If someone has a really ugly, MS Paint-looking icon, my immediate reaction is to think that they're probably a douchebag. Or . . . not so much a douchebag per se, as just sort of generally gross personality-wise. I'm not entirely sure why that is.
I guess maybe it's because I can't figure out why anyone would do that. I mean, LJ's pretty fannish; I think you could find icons for pretty much any character or pairing you want. And even if you can't, surely you could just take a screenshot and crop it down to size without pouring buckets of random comic sans text in a clashing color on it . . . right? In other words, to me having ugly icons suggests a personality that doesn't stop to think through what they're doing.
Also, honestly, maybe it's just me, but in my experience most of the people I've known who had ugly icons really did turn out to be assholes.
People who have personal websites that are hideously designed irritate me as well. I don't mean websites where the html and css are all fucked up (although that would be annoying too, in a different way), I mean people who are clearly basically competent to design something, but for some reason choose to do so in the most aesthetically displeasing color combinations imaginable. . . . So basically, websites that look like something I would expect from a Geocities page circa 2003.
I also judge people who have icons with their username written on them. It always seems so incredibly pretentious to me.
Also, apparently I'm a big girl now, because I always get this ridiculously opinionated about tiny things in the week before my period.