Life is full of uncertainties. Our job is to assign probabilities to outcomes and use those probabilities to make choices. I don't think it's unreasonable for that piece of evidence to increase the probability of his disagreeableness in someone's mind.

Why is it shallow? It's a facade he has deliberately chosen to present to the world. That choice implies things, and he's either aware of that fact or utterly clueless.

Note that I'm not saying anything about what conclusion I draw from that fact, just that it's a fact.

Because that line of thinking implies there is a link between style of expression (external presentation) and character values (morals etc).

It's like judging someone's character by how they dress (e.g. suit == good, dependable etc).