This sentiment, while important to remember if you ARE a perfectionist, also sounds like a really slippery slope similar to something like eating right. "Well if I just eat when I'm hungry" you'll find you're frequently "hungry" if you're a certain type of person. That said, if you're a certain type of person, "not being a perfectionist" could be a great avenue for never pulling yourself out of your slump.

"I messed up at school" Well, I'm not a perfectionist so that's fine I'll just keep at the same pace. TURNS OUT, that pace wasn't so useful. "I am failing Chemistry" OH WELL. I'm not a perfectionist so it doesn't matter.

You can see how this can be a problem as a mantra to a certain type of person.