It may not even be a conscious decision on their part, but people do treat you differently based on their perception of you. So you should take interest in how you present yourself.

PERCEPTION IS REALITY

Their perception in turn shapes their behavior i.e. the way they treat you... and that matters. The better they treat you... the easier, the more enjoyable your work as a restaurant server can be.

This is in your control. You really don't need to invest in the nicest clothes and salon makeovers. Most important is that your clothes fit well, and you look clean.