I've worked as a waitress. Should I indicate this in my PhD application? How will that affect my application?
That sort of employment is unnecessary to include and will be of no benefit if you do. I suggest only doing so if you need to account for a long gap. Even then, you should say why that was necessary to do. And, if you are in mathematics, as the "handle" implies, write about anything you did that was productive in that field.
It might even be better to describe it as "short term, non-academic employment" without describing its nature.
Most people will find this sort of employment irrelevant, and those that don't would be more likely to consider it a negative. You want your application to be positive-focused on how and why you expect to be a success in doctoral studies and thereafter.