Writers that make characters resemble themselves isn’t bad in and of itself, it’s writing what you know. But to some, there is a line between it and Mary Sueism. For those who don’t know, the Mary Sue character is generally an idealised author surrogate but also an idealised audience surrogate if Arrow’s Felicity Smoak’s any indication.
They could have flaws but it’s either glorified or hardly reprimanded by other characters. That’s where the difference is because such projection and the feeling of living vicariously is painfully blatant. Power Puff Girls is a cartoon that I watched growing up and there’s some damning revelation about one of the characters in the 2016 reboot (there was an earlier anime reboot that I also watched) being a blatant self-insert.
One would consider him to be parallel to Anita Blake from Laurell K Hamilton. I haven’t read the latter but it’s been said that the character’s still heavily based on herself but also her lovers in a way. Maybe not unlike with this fiasco where some commentators suggest that one writer may have been infatuated with one of the characters before.
That could raise the bar for avoiding Mary Sues.