Grampa falling in love with Selma was a bad enough idea to start with, I thought, we didn't need to see them break-up almost immediately after.
Bart and Lisa's subplot was funny in a little kid way, which does not make it seem right for this show.
Overall this episode was just plain ridiculous, and not in a good, funny way. I just wanted it to be over when I first watched this. Just terrible I think and not funny at all, except for maybe on or two amusing lines. My overall grade for this episode is a D+ or so