hmmm, I definitely agree that if people ask for criticism people should give it, however I'm not so sure about giving criticism when it isn't asked for. Everyone has different things they want to get out of their art, some really want to improve and criticism will definitely help, but for others it's just very casual or a way to relax and you can't really know which category people are in when commenting.
In my personal experience, tapastic has been pretty much the nicest and most supportive community I've been a part of and I'm grateful for. I think this kind of environment is special, especially for budding comic creators. Making comics takes SO MUCH effort and it's easy to want to give up sometimes, but this community is so supportive and kind that it can keep one's motivation up.
Looking at a community like webtoon, which although I don't think is bad, most people are really nice, but there definitely are more blunt people willing to give criticism, I think can be demoralizing. I've seen some comics with art that could definitely be improved, and the comments on some of those were kind of harsh. Looking back on myself, if I had been in such an environment when I was just starting out, I think I would come to the conclusion that it wasn't worth doing comics.