Is "weekend warrior" derogatory?

The term is generally not derogatory, but, I have heard it used in a negative context.

I work in the medical profession and have often overheard the following conversation:

ER physician: Patient is a 44 year-old weekend warrior who tore his Achilles tendon playing basketball in the park.

Orthopedic surgeon: Have him follow up in my office after he gets an MRI. We'll evaluate for surgery.

In this context, weekend warrior isn't being used in admiration, but rather as an endictment of overstepping one's level of fitness with resultant injury. The tone of voice with which it's delivered is what adds the derogatory connotation.

In my experience, whether it is derogatory depends on context.

Earlier in my life, I was very much into a variety of sports (biking, climbing, martial arts), typically practicing whatever I was into around 5x a week. When I heard the words "weekend warrior", it generally was not used kindly:

"I can't stand riding the trails on Saturday because all they are all clogged up with weekend warriors, falling off their bikes every time they hit a pebble."

That's slight exaggeration, but not too far off. Within the circle who of those who take the sport seriously, weekend warriors who call themselves athletes are generally not held with high regard.

However, at this point in my life, I have a family and a career. I still do sports I like, but at a much lower frequency. I will happily refer to myself as a weekend warrior, because it's an accurate way to describe my level of participation (assuming you are okay with calling sports war).

Actually, no dictionary defines the expression as derogatory, for instance, from AHD:

weekend warrior n. Informal

One who enthusiastically engages in an activity or hobby on the weekends outside of regular employment:

• "Despite the dangers, wreck divers are typically weekend warriors who leave families and jobs behind to test themselves at two hundred feet down" (New Yorker).

I've never encountered it used as a derogatory term. Usually it identifies people who give extra attention to something outside the work week to make up for "lost time".