I dont think there is anything wrong with a waitress asking that! its a perfectly acceptable question. 15% should have been 11.00. Even if it was only a few dollars under, its not a the standard tip for good service. You didnt mention any problem with the service, so I dont see why you wouldnt want to tip her a decent amount.
If a table under tips a server, why should she NOT ask if she did something wrong? Wouldnt you rather your server know that there was a problem and what she could do to work on it, then leaving her to think your just cheap? I VERY MUCH appreciate a table telling me what I may have done wrong while serving them so I'm sure to work on that aspect of my job.
Where I work, if you had left 5.00, I have to tip a bus person 15% of each tip (or total of all tips) and the bartender, which would personally leave me with only 3.50 off your tip...which is kind of depressing on a bill that totals almost $80.
Edit: She didnt confront her for a bigger tip. She simply wanted to know if there was a problem, since the undertip kind of states 'service could have been better'. I dont understand how a server wanting to know what she can do BETTER next time she serves you is wrong. I've asked a table before if there was a problem, and I wasnt doing it for them to feel bad and fork over money. I wanted to know because its my JOB and I want to make sure I'm doing it properly. Waitresses arent anymroe money hungry then people with a 9-5.