At her core Twilight, beyond being “introverted”, is more likely very task oriented and her constant reading and excitement over new things to enjoy showcases lots of passion. At the beginning she valued results and tasks more than friends and it’s clear her loneliness doesn’t go in a typical introverted fashion, but rather she’d rather exceel at her passions she let the bonding with others as a loose second concern at first.
Also, Twilight’s insecurities mostly come from believing others would feel dissapointed in her (believing Celestia would not allow her to become a magician as her student, believing Equestria would fail if she doesn’t act on it, constantly feeling annoyed that her friends and foes would ruin everything), she’s otherwise confident in herself (when dealing with tasks on her own without direction she can be quite a quick thinker and good at applying her knowledge).
She’s definitely heroic, but rather than a typical idealist she’s more of a pragmatic strategists, perfect for leadership an inspiring roles, but not usually what you’d expect for equal term relationships for the most part (granted, not in a concient snobby manner as she remains healthily humble, is just that she’s often more on the logical side so she’d be more of a problem solver and delegator).
And also, Twilight is very outspoken about her ideas, if not about her own personal life, confortable doing whatever task she finds interesting, and most importantly: she’s an activator, not a reactor. When a problem comes in she’s quick to step to solve it, but she’s not above looking for new things to do if possible specially in slice of lifes. (She felt downright dissapointed the map didn’t choose her for an adventure a couple of times and also that she couldn’t attend the Daring Do convention).
I think Twilight is a task-oriented ambivert with a bigger emphasis on the former trait, which might explain why I connect with her so easily, greatly observant, constantly on the move or learning new stuff, admittedly prone to be a workaholic. And also let’s be real, it’s almost an stereotype that smarties=introvert for a lot of people, no matter if that’s not really the case for a lot of people, which makes people quickly put Twilight as an introvert because potato.