What
I'm not underselling what Dak did, he was efficient and had one of the best rookie seasons ever. But he had the league's best OL, the leading rusher, a HoF TE who despite being 100 years old was still somehow always open, a solid slot guy, and Dez who was still commanding double teams even tho he wasn't what he once was. And Garrett sucks but he's a mediocre coach who can still win games.
Mayfield had the worst coach in NFL history along with a terrible OC that basically spent the first part of his rookie season sniping at each other while the team collapsed around them. He started on the bench despite being the #1 pick, and in his first NFL game he led his team back from 14-0. Both his HC and OC were fired midway through his rookie season. His best receiver was Jarvis Landry, who while solid, is not a threatening receiver on his own. All of his other receivers got injured or had numerous drops, alongside a solid but not spectacular running game. And with all that, he still set the rookie TD record while missing the first two games.
Dak may have statistically had the better rookie season. But Mayfield had the best rookie season I've seen considering his play, the situation around him, and the franchise he was sent to. The Browns have been the biggest joke in the league for the past 4 years, but really the past 20 years, and in a single season he's got them looking like they might contend for the division next year