Not grooming is a sign of illness in most cats. They simply don't have the energy.
However, I did know a cat once, a little orange DSH called Skimble, who was a bottle baby, not raised by a momcat. He didn't know how to lick himself clean; there was a female chocolate Lab who used to lick him clean as a kitten, and then when the Lab went to live with a family with a yard (as opposed to an apartment), Skimble had to have periodic baths whenever he started smelling 'catty' (NOT a good smell!).
I've also seen elderly, fat cats who simply don't have the mobility to clean themselves thoroughly. There was a 14 year old Persian mix in 10th Life Shelter when I volunteered there; she had to be periodically clipped because she was too old and too big to reach all of her fur; and it didn't help that she hated to be groomed. (I SO wanted to take her home.)