The problem with the mental retardation portrayed in the film
Is that the way the two characters talk as though they have down syndrome. When they do not have down syndrome
They have mental retardation, but not a syndrome. The actors imitate he Down's way of talking and behaving and it's actually absurd.
Without the specific "accent" and certain mannerisms it would have been a better film. They're both high functioning enough to attend a community college so the retardation must only be mild.
The film makers and actors were clearly clueless about the behavior an IQ below 70 results in.