@Latecomer
I don’t see her as a very screwed up person. In fact, in light of how honestly hard her life has been, being a bastard in the literal sense, who has to support herself, who lives in a wagon, who was demonized, rejected, bullied, and reduced to homelessness and poverty for an event she wasn’t even responsible for, and has basically spent the rest of her screen-time trying to recoup from that while having more on-screen bad things happen to her than any other character, she’s an incredibly good and well-adjusted person. Frankly it makes her a hell of a lot stronger at heart than characters who have done monstrous things in response to minor sleights. Sure she’s obnoxious and a narcissist but of course someone who’s lived the life she has would be, and yeah she’s jealous to a fault of Twilight Sparkle but are we REALLY blaming the character I just described for being jealous of the character who was born with immense power, given a place at Princess Celestia’s side because of said power, has loving parents and a whole family, everything in her life paid for, was given a free home to live in on the proviso she go learn about friendship, and has anything she ever wants just given to her. Really, the question isn’t “Why is Trixie such a jerk” but “Why isn’t she a HELL of a lot worse given her lot in life and what has driven other characters to villainy?” :P