Americans typically don't like healthy, inexpensive or environmentally friendly things in general. They have unused gym memberships, drive cars they can't afford and are drowning deep in medical debt yet still find time to ostracize people for taking their health and finances into their own hands by buying a bicycle and commuting to work. Regardless of what you wear or even what bike you ride people will still "voice their opinions" because you're not conforming to the status quo. I started out riding a Schwinn 10 speed with jeans, t-shirt and a backpack and now I ride a modern bike with racks+panniers and cycle specific clothes and response I receive has always been the same:
"You can't afford to take the subway?" (usually the same people who complain about how unpredictable and expensive the subway is)
"Why don't you just buy a car?" (same person couple sentences later... "OMG the traffic in NYC is so terrible and parking/insurance is so expensive!")
"It's not safe to ride a bike on the street" (but its safe to sit on your couch and eat yourself to death)
"You're such a hipster!" (usually in a condescending or derogatory way...because taking your health and your finances into your own hands is a bad thing)
"[after mentioning that I intend to ride my bike to location X] Oh I can give you a drive" (because sitting your car for 2 hours while you drive in circles looking for a parking spot is better for me)
the list goes on...
One thing I've learned since becoming a bike commuter is that not to give a f*** what people think. Ride your bike, enjoy yourself, save money and get in shape. If all you have a 25 year old MTB then throw some slicks on it and enjoy yourself. It's your life, don't let the opinions of strangers have any bearing on how you live.