Peyton in 2015 was maybe the worst QB to win a Super Bowl. Look at his game log:
https://www.pro-football-reference.com/players/M/MannPe00/gamelog/2015/
The highlights....
- Played 9 games to start the year. Over those 9 games, he had only 1 game with a passer rating over 100 (week 3 against Detroit, when he was 31-42, 324 yds, 2 td, 1 int, 101.7 rating).
- Over those 9 games, he completed 59.9% of his passes, threw 9 td and 17 interceptions. His passer rating was 67.6.
- His last 6 games, he completed 57% of his passes, threw 4 td and 14 interceptions for a 57.3 passer rating.
- His last game of that 9, he went 5-20 for 35 yds, 0 td, and 4 int. After that he was benched until week 17.
- His playoff run featured zero games with more than 57% completions, zero games with more than 222 yards, and zero games with a passer rating over 90.1. His playoff stat line was: 51-92 (55.4%), 539 yds, 5.9 y/a, 2 td, 1 int, and a passer rating of 75.4.
Other QBs have put up similar numbers while winning, but when you contextualize those numbers for the era in which he played, it was easily the worst QB performance to ever win a Super Bowl.