I think people familiar with musical theatre are familiar with Fella, "Standing On The Corner," "Joey, Joey, Joey," "Big D," "My Heart Is So Full of You." Which is why I said it wasn't technically "obscure". It is just mostly forgotten.
I don't get the Follies comparison. Are you saying that it, too, is obscure? Or it hasn't received the respect and success it deserves? I'm saying staunch Sondheim/Follies fans pretty much always say it hasn't received the respect and success it deserves. Personally, I don't care for the show. But I do think when it comes to any art form, many of the most respected pieces of art were not popular, successful or respected when they premiered. Often, it takes a long departure from current fashions and trends to properly respect what had been formally dismissed by critics and the general public.
But no, I don't think The Most Happy Fella is "mainstream", at least not in terms of the general public, who are probably familiar with "Standing on the Corner", but have no idea it was from a musical at all.
"What can you expect from a bunch of seitan worshippers?" - Reginald Tresilian