Jack of all trades is a figure of speech used for a person who is good at doing all sorts of work but isn’t master of one. Its usually said “jack of all trades but master of none”.
Is it bad to be good at everything when someone out there is the best at one. Well I guess a person is his own worst critic. People have tendency to convince themselves that someone could be better than him. I can do number of tasks and I’m good at them. So it called being an all rounder or jack of all trades. All rounder sounds as positive as negative hack of all trades sounds.
But if we look in broader sense being a jack of all trades is not also a common trait as being best at a certain thing is. Sometimes, the thing you feel you are really great at may not be felt by other person. Here is the place where ego and criticism meet. Or maybe it’s just confidence.
Being a jack doesn’t make you good for nothing. At least one can do several tasks at average level.