That was a joke, but I'm happy you decided to take it as if it was a personal attack on you. Personally I don't know how non-chem programs work but provided you're safely dressed everyone couldn't care less what you are wearing in lab. Most of the people you're working with will be from out of the country and will not be anywhere close to "recent" or "fashionable" in terms of how they dress.
What I said is what you would hear if you had asked a prof in the field. Just don't be a total slob when you're TA'ing a lab and you'll be fine. Even then, there are some great people in my group that dress...less than ideally as far as tidiness goes that would get away with it due to the quality of their work. There's a good tradeoff really between how good you are at the field and how much your already asinine TA duties can be kind of turned away from.