Is it bad to play with your cheeks puff out like a blowfish?

Whats a relaxed embouchure???

hold on, I play very relaxed, and, as a result, my cheeks puff out a bit when I play at times...if a beginner does this, it results in a embouchure without control, so that is what is to be avoided..

I concur, I never want beginners to puff their cheeks, problem is they try to imitate sometimes, my kid does that on trumpet as well...

As a semi-professional player for about 20 years I am always amazed at the readiness of some people to prescribe to others, especially in the case of the trumpet/cornet where so much depends on an individual's particular facial and muscular structures. When I think back to my early struggles with the trumpet, it took me some years to find my own adequate embouchure, mainly because I was trying to follow the rules set down by other people. In fact I eventually found that by ignoring most of the rules most of the problems I was having disappeared in a short space of time. One of the important rules I ignored was the rule against puffing the cheeks. In fact, I found that having tight facial muscles and heavy lips demanded I relax the face, the lips and everything else as much as possible and simply allow the blown air to shape an embouchure, all of which allowed me for the first time to gain the full range with a good sound in a relaxed manner. Mind you, this is only a slight puffing of the cheeks that would not be particularly noticeable to an observer, and I can always feel my cheek and chin muscles working despite this relaxation. I would suggest there is a fairly obvious moral to this and similar stories, i.e., let's say no to fundamentalisms of all kinds!