This movie tweaks the 80s as a kind of dumb and dumberer decade, with 80's relics like acid washed jeans, Vanilla Ice, Devo, and Bob Saget (whose part consists of screaming the same four-letter word over and over). There are some good moments with always-terrific Eugene Levy as the corrupt principal who wants to embezzle the money that is supposed to go to the special needs class so he can buy a condo in Hawaii. SNL's Cheri Oteri has some funny blank looks as his game but addled lunch lady co-conspirator. But the only newcomer whose career will probably survive this movie is Eric Christian Olsen, as Lloyd (the character played by Carrey in the original). Olsen does not imitate Carrey; he more or less channels his physical elasticity and dumb-but-thinks-he's-got-it-all-figured-out look, and he adds his own goofy sweetness, creating a real presence in the midst of what is otherwise close to a complete waste of time.
No one expects greatness from a movie called "Dumb and Dumberer." It would be dumber, dumberer, and dumberest to expect much by way of humor or plot or character or energy. Even so, this manages to be disappointing. So, those who fondly remember the original Dumb and Dumber, starring Jeff Daniels and Jim Carrey, should watch it again rather than sit through this lame prequel, showing Harry and Lloyd in high school back in the 1980s.