Fair. Although I disagree strongly.
I don't understand why Metal Sonic being the only member of Team Eggman to do anything meaningful is treated like a feature and not a flaw. To be clear I don't dislike Bowser vs Eggman because Eggman lost, rather I don't like it because if I wasn't a Sonic fan and watched this episode I would actively think nothing of my favorite characters aside from maybe "that Metal Sonic guy was slightly cool I guess" and it's to the point where Eggman's team is just a set of props to make the Koopa Troop look cool.
For instance
1. Eggman - Never comes up with a strategy more complex than "just hit Bowser harder". His mechs lose constantly and he's both always running and he literally needed to be saved from dying twice by Metal Sonic. Then at the end of the fight he was going to fire the Final Egg Blaster right on top of himself which is literally a death sentence instead of first trying to escape like he tried to do later. Then at the end of it all they had Eggman attempt the most pitiful escape attempt ever which I might have been fine with if the entire rest of the episode didn't portray Eggman in the most underwhelming light possible.
TLDR: This is not my Eggman, this is Doctor Evil from Austin Powers wearing an Eggman skin suit.
2. Sage - She does two, technically three things this entire fight and all of them fail. First she tries to block Fury Bowsers attempt to kill Eggman with a barrier but said barrier is shattered and Metal Sonic is forced to pick up her slack, then she throws random Badniks at Fury Bowser which Kamek blocks immediately and finally she fires the Final Egg Blaster which let's admit it anyone could have done and it somehow literally does more damage to her own team than it does to the opponents. Then at the end of the fight she literally joins the Koopa Troop and isn't visably bothered by the fact that they literally killed her father.
On that note I also despise that Sages relationship with Eggman is treated with vastly less respect than Bowsers relationship with Bowser Jr. With Jr we see him back up his dad at the crashed wedding, he reacts with fear to Bowser being in danger and proceeds to be the one to save him by turning him into Fury Bowser, Bowser Jr takes a shot from Metal Sonic for Bowser and even at the end he's the one to stop the Death Egg and let Bowser win. Compare that to Sage who's biggest interactions with Eggman are two nods and a "at your command" before she literally proceeds to have no reaction to Eggman being dead and her joining the enemy team after Eggman's death is played as a "haha Eggman is a terrible leader joke".
TLDR: The difference in how Sage is treated compared to Bowser Jr reeks of bias to the point of being outright disrespectful and the episode treating Sage like she's just another minion in Eggman's army is far and away what I loathe the most about this episode.
3. Infinite - Self explanatory. And look I get that there were to many characters for DB to give spotlight to everyone but having Infinite go out to a Thwomp is plain spiteful especially when members of the Koopa Troop are removed from the fight in the most kids gloves ways possible while Team Eggman get crushed, vaporized, punched into a power block that explodes or end up joining the Koopa Troop. Another thing I dislike is that despite being warned about the suprise attack Bowser was still too slow to react to Infinites attack but he literally suffers no meaningful consequences for it aside from afew background characters before Infinite is dealt with immediately.
TLDR: Infinite was poorly written in Sonic Forces and Death Battle took the easy path by making Infinite poorly written with a spite death when they could have done better.
4. Orbot and Cubot: This is the last and most simple one. Have you noticed that they are portrayed as the most loyal members of Eggman's army in the fight animation only for Death Battle to immediately take it back by having them eagerly join the Koopa Troop. Once again the Koopa Troop are glorified to the extreme aside from King Boo but with Team Eggman even with characters that have genuine bonds (Orbot and Cubot they always do their best to help Eggman and come back to him even when they don't have to while Sage literally sacrificed herself in Sonic Frontiers for Eggman's sake) Death Battle cheapens them both in general and for the sake of a joke.
TLDR: This is my second least favorite Sonic related Death Battle behind Mewtwo vs Shadow and that is a scolding hot take hill that I will die on. Although technically I should dislike Mario vs Sonic 2018 for the same reasons but honestly I just find the disrespect funny in that episode because it is mutually applied to both characters so it gets a pass.
I feel as if some of the issues you have were caused by the fact that Death Battle was on a strict time limit. MORO, the animator, actually had more he wanted to add to the episode's fight, but couldn't due to time constraints.
There just wasn't enough space to give everyone their moment, as the criticisms you have for Eggman's side can also apply to Bowser's. King Boo stopped Infinite and then got cooked by Metal Sonic, Kamek took the phantom ruby and blocked Sage's attack just to also get 1 shotted by Metal Sonic, and King Bob-Ob existed to literally just die. No one besides Bowser Jr. (Who got cooked by Metal Sonic too) and Bowser actually did more than 1 meaningful thing, and that sucks.
For the Sonic side, I do get part of the criticism of Eggman. He was honestly really underwhelming when it came to holding his own, and I did want to see Lightman or Nightmare Eggman, just something where he can fight individually. I didn't like how whenever he fought with his mechs, Bowser always just instantly overpowered him. Unfortunately, as stated before, there just wasn't enough space in the video for him to show off what he was capable of with his own mechs.
But I do get the portrayal of him, like cmon, the great, amazing, and oh-mighty Dr.Eggman having to use his brilliance on some stupid turtle in his way? Preposterous! He's arrogant, cocky, and egotistical just as much as he is an absolute mad genius. And just like in canon, these traits ended up biting him in the ass in the end, with his arrogance leading him to desperately firing the final egg blaster in hopes of taking down Bowser, as he would never admit defeat to this utter buffoon. His ego and lack of care for his inventions led to him hurting his army just as much as Bowser's. He ended up with nothing left in the end to escape with, his eggmobile destroyed in the blast, due to his own self-absorption and underestimation of someone who isn't named Sonic the Hedgehog. The portrayal of his character is near accurate to the games.
Now, Sage's performance does make some sense, as she just didn't have as much to work with when it came to fighting and her overall character compared to Jr. He's been in many installments of games, unlike her, who's only been in 1. Even then, she still had more lines and did more than everyone not named Bowsers, Bowser Jr., or Metal Sonic. She straight-up killed Bowser in his Fury form with the final egg blaster, and the only reason it seemed pointless was because of Bowser's ability to revive, which is an important aspect of why he won.
As for the writing of her relationship with Eggman, they don't have as much depth as the relationship between Jr. and Bowser, with them only having 1 game. There's more weight in the latter with over a decade of showings for their father-son relationship in comparison to the still new and developing one of Sage and Eggman. They just aren't close to being on the same level of closeness and familial love for each other. Bowser and Jr. genuinely love each other, while Eggman is still trying to understand the small, developing familial feeling he has for someone he first considered just an invention.
Orbot and Cubot, I can't comment on them since I don't remember much about them, so I'll give you that. I will also agree on Infinite, Death Battle was definitely just riding the "Infinite is nothing but a bum" joke the Sonic fandom has on him, and it did feel biased against him.
TLDR: Deathbattle didn't do badly in portraying the content of their character, but should have given everyone more to do in the actual battle. Basically, a fight of this magnitude deserved far more time than the usual death battle, and the lack of it hurt everyone.