The meaning of a word in its use. The word "loli" refers to the young looking females that have petite frame. By itself, it's harmless, like every word by itself is. How it is used and where it's used makes people react in different ways. For a typical reader or an author, it's just a popular trope that is used because it attacts eyeballs and reads, because in context of fiction, it's used for the reason and it's fine.
But, when certain amount of people, who treat fiction like reality and reality as fiction come around, and start proselytizing about dangers of X that is only applicable to reality into fiction, the context changes. For them, it's "dangerous" or "enabling" in such a way that it makes real life worse. But, as I said, those people treat fiction as reality, instead of fiction as fiction, and because of such context, it's "bad" or whatever disapproving words they throw out into the wild when seeing such context.
The meaning of a word in its use, but sadly most of the people forget about it when discussing such things. Loli, by itself is a descriptor, and descriptors job is to describe. The context around which that trope is, is what makes the trope to be seen as "good" or "bad". Use if you must, don't use it if you want, but by itself it's completely neutral.
