Agreed. I think the joke the OP posted caters to maniacal positivity though... "if I just think positive everything will be great! That doubter over there is bad at her job because she's negative," and that's so funny (?). For a joke about optimism and not being negative, it's really negative that the woman passive aggressively insults the hair-stylist at the end... unless that's the joke.
Oh well.
I'd like it if after all that, the exchange continued something like:
Hairstylist: "I'm mortified!! I thought it looked great and not bad at all!"
"I'm kidding. He didn't really say that. I just thought I'd give you taste of your own medicine."
Just so the "positive" person doesn't end up a total jerk at the end. But then I guess it's not so much a "joke" anymore after you take out the insults.