Not him but the point is only trolls use the term Mary Sue for canon characters because the single most important part of the original definition, maybe the only important part, was "takes the focus away from the canon characters" or "the plot focuses on them" in fanfiction.
In a genre story, a Mary Sue just describes a general genre fiction character/protagonist. Like James Bond fits because he is super famous, has a million girlfriends, all the other characters that appear are fixated on him all of the time, and he can do anything.
In this case, Rose or Steven have that role of the other characters and plot orbiting around them and having lots of abilities. But like James Bond, that's just their role in the story. Arguing Connie is one is retarded. The plot doesn't focus on Connie. Connie didn't single handedly end the Gem war or lead the rebellion or reconcile with the Diamonds. She's just some townie girl Steven knows. "I think she's too useful despite being just a human" was never the definition of a Mary Sue. Saying Rick Sanchez or Batman or Sailor Moon are Mary Sues at least get closer to the spirit of the idea, but again, this is why only trolls use it for canon characters instead of just describing the actual problem with the character.
But in this case, it's just shippers from Tumblr butthurt that their Tumblr Steven/Gem ships will never be canon. Even if Connie died, Steven, a kid, wouldn't end up with any adult women on the show, he'd just end the show single. Write whatever fanfics you want and make whatever fan art, but get a fucking grip. It's not the end of the world because the writers don't kowtow to your fan crack ships. Every fandom needs to hear that.