If you state "his pants were low enough that you could see his underwear" it would come across as you didn't like seeing his underwear.
Wearing a uniform only means appropriate for the job and clean, not really a uniform. Young guys wear the hanging jeans. I wouldn't say it in a mean way, but I'd say it. The question is: does this FF have dress requirements? Being the OP noticed and had to wonder if he/she should report it, is telling.
I can't really see where it fits in with the questions. The only question I see about uniform or appearance is quite specific - were they wearing proper shirt, apron and no nametag? Which the answer to is yes.
The question regarding name tag and shirt would be answered "Yes," as OP stated. If the employee worked in any industry other than food service, I'd let it go at that. But because he was noticeably and frequently adjusting his pants, I would write something in the comments. Usually, when underwear is worn above the pants, it's a planned fashion choice and doesn't need constant adjustment. This employee sounds disheveled, which as a customer, makes me wonder how clean those pants are. What if he didn't wear bus pants that day? Or worse, what if they are his bus pants?
I did a food shop recently. The answer to the question regarding hat, name tag, uniform was "Yes." Although there was no question that asked "Did the employee refrain from fluffing and swinging her ponytail over the prepared food and not wash her hands before serving the food to you?" I included a comment to that effect because in food service, I think it's important for the client to know.
They are pants that are worn over your own pants when taking public transportation so that you don't pick up germs from where others have sat. It's not really "a thing." It's a reference from The Big Bang Theory [www.youtube.com]@ceasesmith wrote:
What are "bus pants"?
When things do not fit directly in with specific questions, I include the information in the "comments to MSC" section. I say something like: I'm aware that this wasn't asked in the survey, however I do think it's important. Please include this in the report to the client if you feel it is appropriate to do so.
I have written to the MSC to tell them this and gotten the response that this information is not required. After seeing that sort of response, I make a statement about how important I think it would be to the client. Now I just get a thank you.