My sister said people would probably like Matt more if he was a vampire. I was wondering if you guys think so too.

no, Matt is good the way he is, what he needs is screen time! This season I actually liked him more because he'd proven he can handle the supernatural stuff while still being human. Think about it, he vervained two powerful vampires, he killed Finn, he broke into Meredith's apartment, stole the journal, jumped in the pool where he almost died and didn't think twice about it so they could save Tyler! That's a lot for a normal human, he definitely doesn't get enough credit.
Caroline on the other hand became awesome! As a human she was just this insecure mean girl that somehow meant well. When she turned she became an independent, strong and confident women. So the change was needed, unlike others, take Vicki for example. Vicki was awful as a human and she got even worst when she turned. So I guess what I'm trying to say is that being a vampire doesn't make a character great... sorry if I bored you, my comments are always too long :O

^Not boring at all :) I actually agree with everything you said :)