Interesting question....
I think that readers prefer
not having a lot of description of the main character (or even any) because then we (the readers) can picture what the character looks like from our mind's eye perspective, which is more accurate than anything the author can provide anyway

.
Kinsey Milhone is a good example... and so is Alex Delaware, Jonathan Kellerman's creation. In fact, I've read EVER SINGLE ONE of both Grafton's and Kellerman's books yet I'd be hardpressed to describe either MC, beyond the car's they drive (80's Volkswagon, 70's Caddy, respectively). Now, on the other hand, if we're talkin' Kinsey's octogenerian landlord or Rosie at the restaurant, I could give a police sketch artist enough of an accurate description such that it could be fed into a facial recognition program. I suspect this would be true of almost any of her readers because Grafton adds detail to the appearances of those endlessly peripheral characters while leaving the MC to stand, and live, on her own.
Likewise, Kellerman's peripheral characters. I could recognize Milo in a mall parking lot, at night, from ten rows away.
And this has got to be a choice: Grafton is good at description; Kellerman is great at description (in my opinion, Kellerman gives the best quick, vivid and mind-blastingly memorable descriptions of any commercial writer out there... I salivate over his encapsulated peeks at what suspects/witnesses look like, what they're wearing, and what kind of furniture they have.) But as for his main character?... who knows... I figure Delaware's picture is the one on the book jacket, the one misidentified as Kellerman.
I'm lucky in that my series has two main characters, and both are regarding the other one in terms of their appearance and physicality, so I don't bother with any description other than what each is thinking, and observing, close third.
As a reader, in terms of commercial MTS fiction, I crave fast paced action and clever or insightful dialogue but also vivid, "you are there" description. If I'm "looking" at someone, I want to "see" them. If I'm in a place, I want to be fully aware of what's around me. I think because we identify with the MC, who is our way into their world, we don't need buckets of description since, well, we already
know them.
I mean, who writes a description of their mother in a letter to their sister? Nobody. Mom is Mom.
I've been reading Elizabeth George for a long time. Lately, she's been mentioning Thomas Lynley is blonde. Of course he's NOT blonde! I've been "seeing" him for decades. His hair is dark! She needs to get over that absurd idea.
Giving j
ust enough description is a true and remarkable talent.