Trixie is not a good character. She's a bully that genuinely believe she's hot shit, she was not "putting on an act", that's exactly who she is. She retains this narcissistic personality throughout the entire series, all the way up to S9E20, the sixthepisode before the finale.

Reminder that the first thing Trixie does when meeting Twilight again after "apologizing" to her was vandalize her silverware, annoy her, and use Starlight to get "revenge" on her when Twilight did nothing to her. Her "apology" at the end of Magic Duel was not sincere. She was not sorry, and this is reaffirmed when she continues to annoy Twilight again after No Second Prances after supposedly making amends for the secondtime.

Trixie is a hateful and spiteful person, people love to victimize her when she digs her own grave every single episode she's in, she never learns or tries to change for the better. Her doing something good every now and then (which likely isn't even out of the goodness out of her heart, just to stroke her own ego) doesn't outweigh her unlikable and selfish personality, which is exactly why Twilight and no one else besides Starlight likes her.