Not the MOST important, but quite important, especially because it gives D&D a renewed jolt of interest every season, and the four seasons have been spread across 6 years, which is probably better for D&D than if there had been new seasons annually since 2016. Vecna is now a household name in the U.S. If even a fraction of the people who Googled "Vecna" this summer end up actually playing D&D, that is still a big chunk of people.
That said, I'm not sure it's more important than Critical Role has been to the growth of the game. It is more important than, say, Community with its two D&D-centric episodes.