I hate this word as well. When I like a story I'll usually make lots of research on the internet so I can know every bit of it, the characters, the universe... I always try to build a good amount of knowledge on said-story. But I guess most people don't do that and tend to watch stories "briefly". Which means they'll sometimes say something wrong about it and me, being the hardcore fan I am, I'll try to correct them. I've found out that the term "Mary Sue" is often used by people who want to criticize some strong character when then didn't get the character and/or the story at all. It's easy, just say "uuuh it's a mary sue" and it becomes a bad character all of a sudden I guess ?? 🤮 People don't take enough time to appreciate stories and character building...
By the way, I think strong characters are very useful plot-wise when used correctly. A character's strenght is just one of many plot-devices you can use. There is nothing fundamentally wrong about that
