Absolutely! But I suppose my opinion is a bit bias. My mother has been a waitress all of her life, and I waited tables for a few years myself when I was younger.
Servers work very hard, and when I was a waiter (about 10 years ago) the hourly rate was just $2.83, and I don't think it has increased much if any since then.
In my opinion good service deserves 20% or more, and even poor service still deserves 15%.
In my opinion anyone who can't afford or is just too cheap to leave a respectable gratuity should not have someone waiting on them. They should be dining at home, serving them selves, and keeping that tip in their pocket!