Not really. Mary Sues are cliches and even grinders have a few cliches.
Where does this idea stem from?
Mary Sues aren't cliches. Mary Sue is a character that, by its characteristics, is supposed to be special, better than others, or basically begs to be center of everyone's attention.
This has nothing to do with following the cliches, although there
are cliches among the Mary Sue elements (becoming highly skilled at a very young age or having an angsty past are examples). Typically Mary Sue characters are highly attractive and/or very powerful, at the same time lacking any serious flaws (that other players would realise).
Mind you, a character that had its entire village wiped out on their own eyes isn't a Mary Sue unless they reveal this part of their past to other characters/players. I don't see what's so Mary Sue about being evil either (unless you're Duraza-sort of evil ;P ). An eager hero can be as MS as an eager villain.
Also, stating fighting monsters is a cliche among grinders is the same as stating pretending to be someone you're not is a cliche among roleplayers. Neither is an overused phrase/idea/whatever to start with. These actions are what the words represent.