I agree that it's not Family Guy's strong suit. I think they can do more serious stuff and tell a decent story, even if the gags-per-minute formula is their forte.
In this instance, I'd certainly agree that the serious tone didn't make for a good episode. However, for me that's because it was handled clumsily, not simply because it was serious. If Family Guy were able to put forward a slightly more balanced point of view, if its characters were more sympathetic, perhaps it could handle episodes like this a bit better and provide some social commentary without being too self-righteous and still being entertaining. Their position on the issue doesn't offend me, truth be told I'm on that side of the argument. I don't like the way they expressed the argument though, or the way they made strawmen out of the other side. In TV Tropes parlance, they
shot the message. It made it an uncomfortable episode to watch.
The Peterang bit was funny, Mayor West was great as usual because Adam West just makes that character's oddities and non-sequiturs work, Peter's suggestion that Lois work a joke into her big speech then laughing at what clearly wasn't a joke...that stuff was good. The rest mostly fell flat with the usual uninspired potshots, jokes that went on too long and lost their punch, etc. I wouldn't call it the worst episode they've done, but the subject matter was too much for a show like this to handle and on the whole the humour wasn't up to scratch. Pretty forgettable.