Are preferences bad? Nah. Is it bad to have an extremely narrow preference and refuse to entertain anything that deviates from that? Could be.
Saying you don't find fat people attractive, sure that is fine. Saying I refuse to date someone with even 5lbs of extra weight on them is kinda shallow. But that is in your choice. Just be aware that you could be missing out on the perfect relationship simply because of a superficial reason.