As to the original question - I'm going to have to go with "Very yes!" I am absolutely influenced to think a certain way about a poster by their avatar. And I think it makes sense that it is so - after all we humans are used to much more bandwidth out of our communication channels, via expressions, micro-expressions, the way someone dresses or wears their hair, their shoes, and even their smell! I think it's normal for us to have an inclination or another based on more things than diction and post content. Even a bland avatar says something about the poster - that they don't want one, and are bucking the trend for some reason.
Just like
linguistic relativity, however, there is no reason for us to be bound by those impressions, particularly if someone's avatar changes a lot, or if their avatar doesn't give off a suitable personality indication. I have definitely noticed though that when what someone posts something vastly different from what I've learned to expect from them, I experience cognitive dissonance which just...makes my brain hurt. "Wait, that's a ...guy???"
An earlier poster mentioned that our avatars are what we wished we were like, and I think that's spot on. As for myself, I used to use a picture of a blue-haired anime chika in all my online activities. Then I dyed my hair blue. And here I am.