I don't know. I can understand wanting to be comfortable when flying - after all spending up to 14 hours (as on my recent trip to Japan) is not the most enjoyable thing to do and comfy clothes do help make it more pleasant.
On the other hand, airport and airplane behavior seems pretty poor and often inconsiderate and treating others with respect should be the norm. I left Japan on a clean plane with carefully dressed crew and arrived in the US at a dirty airport with behaviors that are just rude. I went into a sort of culture shock.
People putting their feet on chairs, people putting the bags that they had on the floor onto a table to hold it ignoring that people might want to eat at that table, people taking their breakfast into the toilet (bleah) and leaving the plane with garbage all over the floor and seats. Never mind the super loud phone conversations and other poor behavior. And cutting in line to get ahead of others.
On a domestic trip in Japan, the person in front of me actually turned around to ask me if it was okay if he could recline his seat - that kind of courteous behavior doesn't seem common in my part of the world for sure. And queue jumping is definitely not the norm.
For the record, I typically dress casually - pressed slacks and nice shirt with comfy shoes which I find works for me - but I perfectly understand others wanting to be as comfortable as possible and what works for me may not work for others. However, unkempt and dirty - not so much - don't really get that. And, disclaimer, I am sort of an old fogie so I guess that colors my views.