The vast majority of them can't even begin to do the thing they are criticizing.

And?

It doesn't take someone with the skills of Tom Morello or Eric Johnson to know that someone is playing an electric guitar in a truly horrendous manner, nor for that matter if someone is playing it brilliantly.

I can't figure skate (hell, I can barely skate), but that doesn't mean I can't differentiate between an award-winning, technically awe-inspiring performance and a routine skated by some shlub who barely manages to land their jumps.

I'm not a published author, but I can certainly tell the difference between  an acclaimed piece of fiction and some Grade-Z piece of crap that at least had the spelling and grammar right but has an absurd plot, cardboard characters and utterly stupid dialogue.

And I'm probably never going to direct a movie, but I can tell the difference between The Godfather Part 2 and Plan 9 From Outer Space.

The "I'm an ARTIST and how dare you criticize my work without demonstrating sufficient skill" argument (and I'll be generous and call it an argument and not what it is, a pathetic whine), aside from being illogical, gets old real fast.