In my opinion, the clothes you wear while practicing aren't important as you are comfortable with them. But I think that the shoes are very important because of the pedals. You can play in a dressing gown or with a t-shirt or whatever you want but in my opinion you must practice with the same kind of shoes you are going to use when you'll play in public to avoid surprises and miscalculations with the pedals. For example, I always try to avoid practicing on slippers, sneakers or socks.