Is taking mobic (meloxicam) for muscle pain bad if you have acid reflux?

Possible. Acid reflux is due to many things, but typically due to loose lower esophageal sphincter allowing gastric contents to enter the esophagus causing irritation. Mobic (meloxicam) might irritate the esophagus but hard to say if it will make it worse. If the reflux is due to stomach issue then possibly it could be making things worse. I would discuss with GI specialist.