The film's real issue: Harry and Lloyd's characters.
In the original Dumb and Dumber:
Harry and Lloyd were equally stupid. They were not the ideal people to blend into an adult social event of any sort. Basically, they were like 7-Year olds trapped in the bodies of two adults. While they were often volatile and disruptive, Harry and Lloyd were mostly harmless and when they were actually harmful or dangerous, it was the result of unintended stupidity. They were mostly two good guys with good intentions. Lloyd has a few moments where he can be manipulative which crosses into the grey area and that is what separates him from Harry, the more gentle innocent oaf. But overall they were normal(but stupid) guys who were just trying to get through life like all of us. This allowed you to laugh at them, feel sorry for the victims, AND still root for Harry and Lloyd. The only evil act in the original was when Lloyd sold a dead bird to the blind kid.
At the end of Dumb and Dumber, I really wanted to see them win some sort of reward for going through all that. But they didn't(which also works) and you sort of feel sorry for them.
In Dumb and Dumber To:
Harry and Lloyd become outright jerks. It is no longer two 7-Year olds in the bodies of grown men, it is simply two half-retard jerks. Their antics and behavior become premeditated and mean. This takes away the audience's ability to feel for them or root for them. Lloyd is especially exaggerated in the sequel as he becomes twice as evil and twice as annoying. There doesn't seem to be a single scene in the movie where Lloyd isn't acting like a goof or a jerk. It was like watching two villains act stupid through an entire movie and then getting away clean at the end. The two twisted pranks that they play on each other in the movie were completely unrealistic, too extreme even for characters who are suppose to be stupid, and worse of all...they were not really that funny.
By the time the movie was over, AFTER they shove two women into the bushes, I was left with a bad taste in my mouth. I wanted to see them get hit by a car or something just so that there would never be a third film.
On paper, the idea of Harry traveling across country to find his long lost daughter could have worked for a Dumb and Dumber sequel. Even the crazy plot twist that the daughter was in fact Lloyd's would have made the movie that much better. But it's the small things that killed the movie. The writers, whether it was intentional or not, turned Lloyd and Harry into twisted parodies of themselves and then plopped them into scenarios that weren't all that great in supplementing their "dumb" routines.