There are only 2 reasons for not using quilting fabric for clothing. One - you hate ironing. Most cotton clothing needs pressing. If you avoid styles that use collars and long sleeves, probably not a problem. My Mom can't stand to see me in an unpressed apron,
but it doesn't bother me in the least bit.
The second reason, as others have stated, are children's nightwear. The problem is not the cotton, but the design of the clothing. If you design the child's clothing to be body hugging with no frills, you make it safer.
Most flame retardants in children's nightwear washes out after 10-15 washings. It is the styling that reduced the number of burning injuries more than the fabric change. In most states, however, you cannot sell children's nightwear that you have made unless you use the stuff made specifically for children's nightwear (all poly). Way better choice than letting the kid sleep in oversized T-shirts (which are responsible for many burns today).
I am in the process of making some capris from quilting cotton. A couple of sewers have recommended using fabric or interfacing reinforcement for certain types of pockets because of their tendency to tear in thinner cotton fabric. I've decided to change my pocket style to one they use in men's bermuda shorts (it ends up in the waistband) to avoid that problem.