I only saw the last 45 minutes of the debate but the feeling I got from it (and from others) was that everyone pretty much ignored Biden and aimed their fire at each other.
Klobo and Buttgag went at it as did Mini Mike and Liawatha but Biden seemed alone on stage.
I think the moderators and the fellow debaters felt bad for Biden—Biden looked like an escaped nursing home resident who got lost on the stage...babbling incoherently...