There's two kinds of results -- raw results (plain stats, W-L record, etc.) and there's expected results (stats compared to other players or to the same player's previous years, similarly for team stats, etc.).
HOF-level talents get wasted or ruined in the NFL all the time. This is just one example of many ... but check out Steve Young's two seasons in Tampa (19 starts). The offensive staff was pretty much run-to-set-up-the-pass guys that figured force-feeding James Wilder 360+ carries a season would help support long-developing pass plays way downfield -- a 1960s approach.
Same thing could've happened to Mahomes. Say he time-traveled back and played on Bum Phillips' Saints. Similar deal to Steve Young in Tampa. OC King Hill was still running the I-formation with George Rogers, Hokie Gajan, and later Earl Campbell. Phillips would be telling Mahomes, "Son, three things can happen when you pass the football ... and two of them are bad." When Mahomes did get a chance to pass, it would be off of play action on third-and-longs where the line would be expected to hold their blocks a little too long. Mahomes could take off ... but in the 80s, defenders could take his head off. With all of Mahomes talents ... he wouldn't have thrived in that kind of environment.
What players need to max out is someone who can support their preparation, point out things on film that they wouldn't catch themselves, catch things in footwork and delivery that aren't obvious to most. This turns Mahomes and Young into Super Bowl winners. It turns Daniel Jones into a playoff-game winner with career-best stats. And it turns Tommy Divito and Tyrod Taylor into guys good enough to tread water for a few weeks.