Isabelle and Mistress T have it right on.
I have had CRAZY sessions with average looking middle aged men with a bit of a belly, just because the energy, mutual respect, and passion were there. And I have had sessions where I really had to work and fake everything with super attractive clients who just lay there like they are masturbating themselves.
A lot of times those guys that just lay there are actually really shy, I find that if I can really put a lot of energy into the first session and they come see me again they are not as nervous and it's much more enjoyable for both the second time around.
However when they are laying there for reasons Isabelle said it does get a bit tiring and it also is hard to keep going for so long. I start to wonder if he is not having a good time and then I feel bad.
But in short, nope, what someone looks like doesn't affect the level of service I give. Sometimes what someone looks like can affect my own personal level of enjoyment but I try to always give the same experience to each client and in the end I've found that compatible energy is more important to equal enjoyment than attractiveness.