As the title says, something happened starting with season 17 that changed American Dad for the worse.
The team working on AD started missing the essence of what the show was. There's no clever writing anymore that really surprises you with its jokes, the characters are too quirky and random, the storylines are not interesting. I have not bothered to check if they changed writers or not, but either way, something happened with the team making the show.
Consider this episode, for example, Fellow Traveler. Do we REALLY need to know Roger's backstory of why he's able to change personas so well? It feels like they're running out of ideas. If you would've just shown him as a crazy, sassy alien on his home planet, that would have been much better. Show us what stupid things he did on his home planet as Roger, because THAT'S where he actually became that way, not on Earth. It's just poor writing.