It doesn't really make me think less of the guy. It depends on the guy and the type of "vibe" I get from him but I would probably think 1 of 2 things.
The more likely thing I would think is:
"Wow she must be rich/she must be f*cking him really well", especially if she's MUCH less attractive
If she's a little bit less attractive I'm more likely to think that he probably just likes her a lot. But honestly, it is unusual to see an attractive guy with an unattractive girl, since guys are more looks-oriented. I mean, if you see a young girl with a much older unattractive man, you know it's very likely that he has money and he's getting a nice roll in the sack for it.
I most of the time think 'what is he doing with her' and then I think 'well maybe she's a good girl with nice personality and get well with him'. But yes sometimes I judge them both why is he with her she's the ugly betty and what is she doing with him is that no stupid kiddos?
i usually think that the guy doesn't think he is as attractive as he really is. it's very rare to see this combination. studies show you usually choose people who are the same attractive "level" as you so that's what I think.