"I have noticed that really good singers and musicians never put down others and are always supportive."

Well, if they're all that great, they're used to listening to folks who are less talented than themselves, and they've had time to develop an appropriate attitude. It can be relatively easy to be gracious when you've achieved a bit of success yourself.

Folks in the fair-to-middlin' category may display their negative reactions not simply out of jealousy or small-mindedness, but perhaps more forgivably, from feeling "I'm way better than that fool, and I don't get much appreciation."

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Very nice statement from Sedayne; I would elaborate thusly:

Many vocalists who lack conventionally "good" voices can be really excellent singers of certain kinds of songs, because they can very effectively communicate the feeling at the heart of their songs. I would suggest that many famously successful folk and blues artists meet this description. Even instrumental playing can be technically flawed while at the same time emotionally compelling or otherwise effective in some regard.

On the other hand, we've all been subjected to performances that are off-key, inappropriate in volume (either weak or too damn loud) AND at the same time completely UN-expressive of any discernible human feeling. That's when it can be very difficult to respond positively, or even diplomatically!