I think people kind of misunderstand what type of athlete AP is/was. He's jacked compared with the average human and he's really cut, so he looks muscular, but from a height/weight standpoint he's actually more of a lean-framed speed back than a rugged power back. 6'1.5" 217 pounds (28.1 BMI) with 4.41 speed and phenomenal explosiveness in the jumps (38.5" vertical / 10'7" broad jump). He's big and strong enough to run hard, but for me what made him a star was his explosive qualities. Not many backs have the elusiveness, footwork, and speed that he had in his prime. That's why the Fournette comparison really doesn't work for me. Whether you like Fournette or not, he and Peterson are not alike in terms of body type and running style.
As for Mixon, his combine numbers aren't great, but I think they're good when you look at the total package. I don't have an exact number on his height, but what I've seen says he's about 6'1 1/8" and 228 pounds. So we're talking about a guy who's not only slightly shorter than AP, but also ten pounds heavier. With that being the case, his 4.50 time starts to look pretty decent. You'd like to see a better broad jump, but the vert is acceptable.
Ultimately, the backs that I'd compare Mixon to in terms of NFL impact are LeVeon Bell and Zeke Elliott. Neither one of those guys really crushed the combine either. They excel by being versatile and generally athletic, but they're not elite in any one category. I think Mixon is a similar type of package.