On a completely different topic: Is Hellavah Boss necessary viewing for Hazbin Hotel (lore wise)?
Not particularly. You learn a lot more about the universe in general (such as the other sins, the other rings, how hellborn work, etc), but none of that is relevant to Hazbin because it takes place exclusively in Pride and deals with only sinners. Basically, if you like general world building, yes, Helluva has a ton of that. If you exclusively want more info on the Hotel or Hazbin characters, no, there's nothing.
A couple of months ago it was revealed that there will, at some point, be a cross over, but what, when, or how significant it will be was not mentioned.
Is it good? I thought the character Blitz seemed too much like a more annoying Roger from American Dad, so I never watched it any further. Is his character less annoying in the main series when compared to the pilot?
The pilot was completely replaced explicitly because the characters changed so much. The original pilot is now considered non-canon and the series starts with
Mission: Zero instead. If you disliked the original pilot that's good, because it is totally different now.
As for "is it good", I personally like it more than Hazbin but it is very contentious. A lot of people don't like it (especially here) because the later parts of S2 focus
heavily on a gay romance (Blitzo/Stolas) rather than I.M.P.'s assassination business.
It isn't like, amazing, but the songs are catchy, I thought the story was interesting enough, and IMO it is worth a casual watch.