Yup! My family owns a restaurant, and I can confirm this is true in my area (northeast US). My sister worked as a server for a summer (2017, or thereabouts), and averaged about $46 an hour. Not to put too fine a point on it, but it helped that she was young, attractive, and witty. When I served, I made about $30 per hour (not-unattractive male). Sex sells, as she used to quip- it's true, there's also a gender-based component to it.

Anyway, most of that in principle was cash tips. She was young enough where she was getting all of her tax back anyway, but I do know many people who underreport the tips they receive.