When people say he's been underwhelming in the playoffs (compared to his supposed stature) it's because he's been outproduced in various runs by teammates like Tyler Johnson (2015) and Ondrej Palat (2022).
Nobody said he's been underwhelming. Someone
has said he's the single worst playoff performer of a generation, which is clearly absurd.
As to single playoff runs... er, so what? Crosby and Malkin were both outproduced by Phil Kessel, a non Hall of Famer, in 2016. In 2013 and 2015 combined (back to back playoffs), Ovechkin was outscored by Joel Ward. It happens. Palat is a good player who had a good playoff run. (Not sure what's the point of bringing up Johnson, whom Stamkos massively outscores in playoffs.)
Since 2020, the Lightning's goals-per-game leaders in the playoffs have been:
0.52 - Point
0.47 - Stamkos
0.38 - Palat
0.29 - Kucherov
0.27 - Gourde
Scoring 0.47 goals per game in the playoffs over 50+ games is
really, really good.
I'm gonna go out on a limb here and suggest that a guy who scored 11 goals in a playoff run, was team captain, and won the Stanley Cup, probably isn't the worst playoff performer of a generation.