In the next Halloween episode, Twilight and Trixie will bond over Trixie's ability to actually comprehend Twilight's costume.
@Apple: Actually, Trixie doesn't strike me as having any of the hallmarks of the truly self-absorbed. Interestingly, she appears to have more of an inferiority complex than anything else. It's actually fairly common for those with inferiority complexes to appear to have a superiority complex - they feel the need to fool everyone into believing that their great. Sometimes to try and fool themselves into believing it as well, sometimes just as a facade, putting on a front they feel people will like. The opinions of others are tied to their sense of self-worth, in that case.
The interesting thing about Trixie is that while she pumps herself full of glamour on-stage, she actively resists attentions off stage. She very nearly shoves Snips and Snails away when they try and show their admiration, and pretty much sequesters herself in her carriage. She doesn't make much of an effort to socialize, much less aggrandize herself as someone truly self-absorbed might.
Her behaviour, on-stage and with the Alicorn Amulet, makes me think at one point she was picked on pretty severely... it's easy for the abused to come to associate strength with the ability to do harm to others, and weakness the lack of ability(
or desire). Put in this context, she may have come to associate strong magic with the ability to show up others, to lord it over them or straight up humiliate them.
When Trixie helps out Twilight's show, she doesn't make much effort to make herself known to anyone BUT Twilight. She still has elements of showmareship, yeah, but she's clearly not showing off. She's actually a decent pony, just... a showmare. On her own, I don't think much of her behaviour would have developed naturally. So I don't think she's terribly self-absorbed. I think that she may have, at one point, believed she SHOULD have been better at magic. Perhaps because she was a mediocre student at best, perhaps because she was bullied... perhaps she comes from a line of powerful unicorns, and is simply not as good as the rest of her family. Whatever the cause, I don't believe she's as self-absorbed as all that.