Lamar is not a playoff performer. When it comes to Lamar being a symbol to some people, a lot are too hesitant to call it what it is but the truth is he's just not built for the postseason...at least for now because he did take a step by making it into the AFC championship game so it could be a sign for things to come in the future.
However just like Dak, they both have good/great regular seasons winning awards, making milestones, etc but come the postseason neither one of them gives you the performance needed to help their teams win in crucial moments...
Whether it's pick-sixes, overthrows, sacks, fumbles, etc they both shrink or overthink the assignment and by the time they wake up the game is too far out of reach or time is running out (Dak against Green Bay for example).
Just like in any sport, legends, and greats are born in the postseason. The reason why Mahomes, who isn't even halfway through his career yet and is already considered to be in the Hof tier in most people's eyes is because of what he's done in the postseason.