People know OF it for sure, but the only reason it’s so long-running is because it’s on an off-beat cable network that requires very little in terms of ratings, and is critically-acclaimed. It has a very dedicated fanbase of about 3-4 million viewers in the US through multiple platforms. That’s perfectly respectable, but far from “mainstream.” It’s more of a cult favorite. The majority of the moviegoing audience has never seen an episode, I can guarantee it.
No show that airs on FX would be considered “mainstream” honestly.