Yeah
Don't even know what it means and can't search because I'm working
People tend to have their own definition for what a Mary Sue is a lot of the time.
For me, a Mary Sue is a character who is, for the most part, represented as nearly perfect, with little to no
actual flaws or weaknesses. They tend to solve problems a lot quicker than other characters, or even other protagonists, and usually do not face any significant problems, and if they do, most of them are solved easily, or do not result in any permanent or long-lasting side effects to the characters. They
may have conflicts that are disguised or presented as being huge problems, but ultimately, do not end up being anything massive or impactful in the long run (Good example is Rimuru losing a lot of his comrades and having his country destroyed by enemies (Presented as a huge problem in the story that could significantly affect everything that's been built up so far), only for all of that to be completely restored and undone after he gets another power up, which hardly leaves an impact on the overarching narrative past this point).
However, at lot of the above can actually be circumvented if a character has other aspects that are focused on. A good example would be Saitama from OPM, who has a lot of Mary Sue characteristics, however, nobody would ever call him a Mary Sue. Reason being the story explicitly showcases how his position as this overpowered, limitless force, has caused him to become somewhat isolated, unchallenged, and lacking in character.
Rimuru, while also sharing many Mary Sue characteristics, unfortunately does not make up for it in the same way, in my opinion at least. There's a lot of wasted potential in Tensei Slime, which is unfortunate because I truly do think its world and concept are interesting.
