Yes, the physical “attractiveness” can influence the responses of test participants. If you appear well dressed, it may make the participant feel they have to be more formal and some may look to you as an authority figure, and either reactions can affect how they respond. If you dress slovenly then some participants may not take you or the exercise very seriously.
A personal example: I don’t consider myself attractive, but I do consider myself reasonably good looking. I used to dress very well when I facilitated tests (suit and tie) and at 6 foot 4 in a suit I can make quite an impression (think Penn Jillette).
Over a couple of tests I noticed a couple of the female participants (and one male) was being… Well, they were flirting with me. Being an unbiased facilitator I had to not reciprocate and I had to “throw out” one of the interviews because the person was… Well, she was pretty blatant.
I was flattered, but I also decided to have less of a “presence” in the sessions – the focus of the test obviously is what you are testing and what the participant does and says… Not on you. So now it’s downkey polo shirts and nondescript khaki pants, to “blend in” and not distract.