The way I see it, it comes down to external vs. internal threat. As campy as the terrible interdimensional creepy things are, they are a clear external threat to the people of Earth. Making Dr. Manhattan the fall guy internalizes the threat as he is-was "of us".

With Dr. Manhattan as the patsy you get some superficial solidarity while every nation tries their hardest to recreate him--to obtain their own WMD+/God. With the terrible interdimensional creepy things humanity faces a clear external threat; Unite, people of Earth, for the Universe will fucking eat you.

Maybe the monsters could've been done in a way that wouldn't have people laughing at their implausibility. Using Dr. Manhattan destroyed the message. The film's ending wasn't worth the sacrifice, IMHO.