The passing TD number for Cam was awful. The other numbers (completion percentage, yards per attempt, yards per completion, etc.) were not good, but within spitting distance of average.
I think the best measuring stick for a QB is the overall efficiency of the offense, and it was bad with Cam in 2020. They finished 24th with 1.92 points per drive. OTOH, they were barely any better with Brady in 2019 (1.97 points per drive, 17th). The 2019 Patriots fell off over the second half of the season, scoring 17 offensive TDs over the last 8 games, a virtually identical rate to the 31 offensive TDs the Pats scored in 15 Cam starts.
Brady unsurprisingly played a lot better with better receivers in 2020. I expect the Patriots' weapons to stink in 2021, but they ought to be better than they were in 2020.
I hope they improve on Cam too, but I'd rather him back than, say, Ryan Fitzpatrick or Andy Dalton.