Dirty Grandpa has a concept that could be funny on paper, but it ends up just being a film that tries so hard to offend you with cruel jokes about "outrageous things" that you just raise your eyebrows and go, "really." It has no believable basis, and you might expect it, but even The SpongeBob Movie is grounded better in reality than this "movie"?
Can it even be called a movie? It honestly felt like a poor SNL sketch that just went on way to long. Sure, there's some funny parts, there's some parts that will make you laugh. But the jokes don't land enough to make it worth it, and the most (to all) of the jokes I wouldn't even call jokes because there's no thought in them. It's al your standard dick, poop, sex, fart, and drug/party jokes. Sure, those jokes can be done well, movies like This is The End proved it, but this is not the case.
Highlight of the Movie? Efron, De Niro, and Plaza do the best they can with what they're given (plus, Plaza is hot.)
Low point of the movie? Efron, De Niro, and Plaza wasted their time. Plus, that Plaza/De Niro sex scene mentally scars you, and you no longer look at Plaza as really hot, you just see her as that girl who had sex with De Niro and quickly want that image out of your memory.
Other than those three characters, the rest are particularly annoying, and there's a few that the film just can't help but keep focusing on.
Overall
Within the movie there is a running joke about De Niro putting his thumb up Zac Efron's ass. That's exactly what this film ends up feeling like when you watch it.
D-