Trixie isn't self-absorbed. She's merely oblivious to her surroundings! Which is why she didn't notice that Ursa Minor in Boast Busters right away! But, when she did, she made sure to stay there, selflessly laying down her life to protect the two morons that had led it to her wagon! Of course, thankfully for the two moronic children and Spike, who had convinced said moronic children to lead it into town in the first place, no one died, but that's because it was a children's cartoon show. They couldn't show what would actually happen if two morons led a dangerous wild animal into town. (That being that people would have died. Most likely the foals and Trixie, unfortunately).