I think the issue is not that either Fallout 1,2 or even 3 (and all other games) have silly humor. I think the issue is that in F1 that kind of humor was most of the time juast meant as "joke" will Fallout 3 for example made it a canon and part of the setting and story. Like the aliens you know which never have been a part of the game and suddenly in Fallout 3 they get a own DLC and as how I heard a few people even said that you canfind clues they started the nuclear war or such crap. In either F3 or Vegas you can not just say that its a randoom encounter anymore or just something you encounter, have your fun (or dont) and move on with the other serious tone or black humor. Well thats my issue with the "new" parts of the franchise anyway. But well ... if people love to kill enclave troopers in their samurai armor while a giant robot which could be the grandfather of Optimus Prime spits some US propaganda from before the war at them ... 
