I think there are two aspects that combine to form the general opinion of Puff being 'gay': Puff can be boring to watch (which is self-explanatory) and Puff is frustrating to play(for some). That frustration is likely because the Puff MU is a very unique one that requires precise and intelligent play, if you run around trying to do fancy tech skill, and don't focus on your defense, you'll form habits, and habits can be rested. Also, when a new player is unfamiliar with the MU, they don't know what to do, for example, If Puff is fighting Falco and lands a low N-air -> grab, you have 8 frames to DI before you no longer have the option to DI, but the average players reaction time is at the minimum 12 frames, usually 15-20. So an experienced player is subject to getting hit by that often. Add in newer players typically having poor spacing, and you'll get eaten up by Puff's spacing.
TLDR: coherent Puff players form one the barrier most players see, where they have to start playing smarter, not faster.
This is around the time when they'll start learning from matches, not combo videos.
AKA Puff beats scrubs.