So, here's a detailed explanation as to why I believe stars are being created in the Mundus boss fight.
First of all, despite all of the claims about it, the area you are in is very clearly not any regular stretch of space. When Mundus first teleports you, it looks relatively normal, with a stereotypical night sky background used. But when the fight actually begins, you are in a completely different environment. There appears to be some kind of... vortex? (Probably not the best description for it, but you get the idea) while what looks like clouds on the top and bottom of you zipping past. I don't care what area of space you look at, it won't look anything like that. So whatever this place is, it's clearly not space.
Second of all, Mundus' attitude. Even with Mundus' depicted confidence, he's acting ridiculously confident in the battle with you. He knows that he was once defeated by Sparda, and going off of the claims made by Griffon that Mundus seems to have witnessed (note how Mundus appears the moment Griffon calls for him, meaning he was within earshot), apparently Dante is even greater in power than Sparda (not necessarily a reliable feat, by the way). Regardless, Mundus would have heard this and known that Dante is no small threat on his own. But then, when Dante goes up against him with his own power, plus that of Sparda, Mundus still acts completely unphased despite this clearly being a battle that he should know he can't win. It's only when Dante activates the Sparda Devil Trigger that Mundus finally shows himself and opens the portal to... wherever. Yet once they both go inside, Mundus continues to act unphased towards Dante. You see what's happening here, right? There was no reason for Mundus to have opened up a portal to nowhere to fight, that's just a waste of energy and gives him no advantage, which even he would know he needs. The only possible reason he would have opened up the portal, bringing Dante inside and then proceeding to act the way he does is if he has brought them to an area where he has a massive upper hand. The only possible upper hand he could get from teleporting to somewhere outside of our own universe (which I showed earlier) is if it's a place where he has control over everything. It is almost certainly a dimension that he created, if not an entire universe (although again, I'm going with conservative estimates here). At the most, in the battle we see large clusters of stars (both very clearly before the battle, and what appears to be stars during the battle). I'd be more inclined to call this a 3-C feat (and it could even be possibly used to justify 3-A), but for the sake of consistency this is also quite well explained as a 4-A feat, which is perfectly consistent with what we saw earlier with the Nightmare feat.
I could go on, and on, and on, but I think this is enough for my point to be clear: Mundus is shown to be at least 4-A.
