A trait I found notably not stereotypically MSish about Starlight is how ponies weren't universally praising her magic, between NSP (Big Mac Incident, Mrs. Cake with her magically made cake), ELTSD, ARP, Uncommon Bond (Sunburst got on her case regarding "pretending to be foals"), and A Matter of Principals (where she scared the students by making it look like she killed Discord right in front of them, and she had to commit awkward reassurance).
You'd generally expect the opposite with a few of those instances, notably with Mrs. Cake, unless they were supposed to be completely unsympathetic, with a stereotypical case.
I generally try to use that term for fanfics (albeit not universally), and even then, not all that commonly, unless it's egregiously bad in terms of "high prominence and/or broken abilities just doesn't make sense and quite possibly overshadowing other characters without good reason."
An aspect of its overuse is everyone has their own personal definition, that you'll notice typically includes traits... practically any main character has.