Answer: Characters should be described as soon as the reader meets them. it's not fair to mention they are blonde 20 pages later after the reader has already imagined a brunette. And describing a character feature by feature can get to sound like a catalog--or an information dump. I like a general description or to describe one or two things and leave the rest up to the reader’s imagination. The key thing is to describe them when they are first mentioned, before the reader’s imagination kicks in with something you never intended.
Using a mirror for first-person descriptions has been done to often, and too often badly, as you mentioned. But having a character think about how they look is fair and can be intriguing in either first or third -- if what they consider faults turn out to be fairly attractive features.
I'm certainly no authority when it comes to writing good description, but anyone can get lucky. Here’s an example of how to use the "thinking about features" approach from my mystery story, “Final Exit." It begins as follows:
"Actually, Jon knew he and his sister, Jill, looked alike. Same dark eyes, same straight nose, same generous mouth. But his hair was dark where Jill's was fair, and his jaw was square while Jill's pointed chin gave her face a heart shape."
We are in Jon's viewpoint, but can see both Jon and his sister are relatively attractive without either of them seeming conceited.