Where'd you get that logic from? Even the synopsis says she has been killing slime's for 300 years. Has the definition of 'Mary Sue' become so deluded diluted that it's not even recognizable anymore?
She's an immortal loli who is the stronk. This is a story about her and her loli harem enjoying the pleasures and leisures of life.
Like Narehate said, don't be a Negative Nancy.