Mattison has looked OK when he was in a complimentary role, but to me he has been underwhelming when he was the lead dog.
		
		
	 
He bombed his 1st start in place of Cook in 2020 against the Falcons, and I think that game has stuck with people, but he's been mostly pretty good other than that. His stat lines in games Cook has missed has been:
2020 Falcons 10-26 rushing 1-4 receiving
2020 Lions 21-95-1 rushing 3-50-1 receiving
2021 Seahawks 26-112 rushing 6-59 receiving
2021 Lions 25-113 rushing 7-40-1 receiving
2021 Lions 22-90-1 rushing 3-34 receiving
2021 Rams 13-41-1 rushing 3-29 receiving
So his 6 starts produced an average of 20-80-.5 rushing, and 4-36-.33 receiving for a 17 game average of 340-1360-9 rushing and 68-612-6 receiving. While that workload isn't likely sustainable over 17 games, that's still pretty solid in my book.
Cook has a better YPC, because he breaks some big plays, but I really think Mattison might be the better overall player, especially considering price tag. Also seems like the Vikings want to be a pass first team under O'Connell, and Cook hasn't been an asset there in the last 2 seasons.
Ultimately, I don't think either guy is anything special, but considering the Vikings cap issues and holes on defense (going from Donatell to Flores at DC is a good start) I think Cook is very expendable, and Mattison is likely cheap to retain, and has had success.