Yes, a bad rapper made a good album. If Kanye West, Group Home, Jeru, and a million other motherfuckers can make good albums, a rapper who used to be good can also make a good album.
I don't know how exactly to break down why I like Purple Haze. It's raw, ignorant, crude. Though it definitely has some baroque production, it's not polished at all. Most of the beats have good bottom to it. I enjoy Cam'ron's lazy delivery and relaxed flow. "Dipset Forever" is a great beat. I find the end of the first verse of "Harlem Streets" and its chorus to be profound in an understated, plebian sort of way. There's nothing on the album to suggest Cam'ron particularly cares to appealing to anyone outside of his core audience. It's a fun record on some dude shit. Clever lines and boasts that you can repeat or laugh about in the barbershop or while passing the blunt in the cipher. It was a good contrast to other hip-hop albums that while good, are overproduced, pretentious, and corny (College Dropout, The Black Album, etc.).
It's all subjective, though. One of the annoying things about hip-hop is that its rejection of traditional music rules has left fans without the vocabulary or mechanisms to evaluate something on a scale that everyone perceives in the same way.
If we're done with Cam'ron, Psyence Fiction? Really? I'd only consider the two "Drums of Death" joints hip-hop (thread idea: "Just because Andre 3000 or DJ Shadow or Prince Paul or Automator or Lauryn Hill or Cee-lo or Whoever Put out the Album Doesn't Mean it Is Hip-Hop" b/w "What the Fuck Is Post Hip-Hop") and I don't even like the Mike D one. The Kool G Rap one and "Rabbit in Your Headlights" are the only songs I can mess with on that.