Mat expresses negative opinions of everyone at some point in the series. And, in my observation, it takes an act of the Pattern to make him change his mind about someone.

IMO, Mat started with a negative opinion about Elayne based on her station in life (I've seen some POV from other Two Rivers characters that were some what similar), and then he succumbed to confirmation bias. He has a huge bias against anyone coming from a noble or Aes Sedai background...and she comes from both!

She never behaves in a 'spoiled princess" manner, but she does behave like someone who just never had to do the sorts of things he had to. That's not her fault and she shows a lot of willingness to learn skills that are "beneath" her station. Cannoli pointed out a few. Yes. It's clear she is from a higher station. That is not being spoiled. For god's sake, the woman stripped down to unseemly clothing and did somersaults above a crowd in a circus in order to maintain a secret identity. That is not a spoiled noblewoman!

Besides. POV is a wretched way to decide how someone acts when you have plenty of evidence to use that's not POV. Jordan gave us a lot of opportunity to see Elayne's behavior through neutral eyes, or at least multiple eyes, in order to form our OWN opinion.

And, honestly, even if we agree she's spoiled, she's definitely not the most spoiled of the girls. I'd say Egwene takes that cake, knife, tray and table on that. Even Avienda can behave a little spoiled, albeit in a different and nonstereotypical manner. The more I think about it, Elayne's probably the LEAST "spoiled" in her behavior.