It's not possible for an NFL QB to hit their ceiling of potential after 1 year in the NFL. It's a myth. Maybe there are caps on potential at other positions due to athletic limitations but QB is not one of those.
QBs who achieve their full potential do so through a combination of hard work, commitment, some luck, and executing at a high level in big games. Those who don't work as hard, aren't as committed, suffer bad luck in the form of coaching/injuries, and/or lay eggs in big games, aren't as successful. It's ultimately a position about decision making. Athletic talent adds value but is limited by decision making. If you're looking for a QB, decision making always trumps physical ability, because without the former the latter is irrelevant. The only circumstance in which you could argue a QB might have a ceiling is if they're extremely limited by their height and/or arm strength. Russell Wilson and Drew Brees are good examples that even that may not be the case.
I hate to keep referencing Brady, but everyone would have said the same (that's he's at his ceiling) about him after the 2001 season. He has 2 inches on Mac and that's it. Brady gained lower-body/core strength and by 2003-2004 onward had a live arm, but he didn't in 2001, and physically he mirrored Mac in a lot of ways. Skinny, weak lower body/core, inability to drive the ball at times. Absolutely nothing about Brady suggested he had much more of a ceiling beyond what he showed in 2001, and yet here we are.
If Mac Jones works hard, adds lower-body/core strength, continues to learn/absorb the game, and gets some fortune of good rosters around him, I don't see why he couldn't be a Super Bowl champion some day. His game against Buffalo was his best of the season, IMO. His arm looked really live and he was throwing some ropes 15-20+ yards downfield. I was extremely encouraged by what I saw from him.
Brady didn't achieve his status via physical ability/a high ceiling. He achieved it through incredible consistency, longevity, working to improve weak points in his game, and incremental improvements year after year. He works extremely hard, studies the game, and those accumulations of incremental improvements combined with being on some phenomenal teams, with phenomenal teammates and coaching, has led to what he has become ... not some abstract notion of a "ceiling" (which is, again, basically a myth -- it refers to a hypothetical potentiality that is irrelevant to what actually happens on the field, which is what determines outcome, legacy, and ensuing perception -- not what one's "ceiling" is).