How this script attracted the calibre of actors it did I cannot fathom. The story is tired, trope-filled and cookie-cutter, with the barest amount of heart and character development to prevent Dirty Grandpa being a total write-off. The acting is decent throughout with some fun moments from Zac Efron, in particular, but nothing that will leave an impression.
The reality is that these are the best things about the film: decent acting and the existence of a storyline. But that doesn't really justify why the film exists; they are the bare minimum a film should contain, which is a good way of summing up Dirty Grandpa. The entire film exists as the bare minimum enabling excuse to have an old man say and enact lewd/offensive "jokes" for just over an hour. Had these jokes been pithy, witty or at least funny then perhaps that would have been enough. Unfortunately, they maintain none of those characteristics, falling flat or coming across as inanity for most of the film. The only moment that ever felt more than telegraphed or shoe-horned was the "flex off" scene, but even this was weirdly broken apart by out of place fart jokes.
At the core, it seems like a writer somewhere wanted to create a story about the pitfalls of spending a lifetime pleasing others. There's some form of moral mixed up in there but it never really breaks the surface and reveals itself. The clearest message, that you should "live what you love" and not what other people want, is somewhat flat-lined in a finale where Efron's character does precisely what his grandfather wants him to do. He quits his job, cancels his marriage and runs off with some girl he barely knows, seemingly due to an epiphany but, realistically, because his grandfather has convinced him that this is what will make him happy, despite Efron's consistent attempts to prove him wrong. Effectively, he breaks free from the controlling clutches of his father directly into the controlling clutches of his grandfather. The women in his life are no better, trading an overbearing control freak fiancee for a free-spirited college-girl who manipulates him into joining her on a year-long boating trip. Does Efron want to go off and photograph climate change for a year? It doesn't really seem so, he just does it because she gives him no other choice. So, what exactly is the message here? As with most aspects of Dirty Grandpa the answer is that there really isn't one, just a vague form of something that might have been.