The state would have to be the one to decide to file or to drop (if already filed) the charges and not the victim. The state may decide to let things go if they don't have a cooperative victim (Unless it's a domestic violence charge.)
But it is unlikely they were dropped because the suspect has a mental disability. A person has to be so disabled to...well a few things depending on the state. But at least so disabled to not be able to tell the quality (as in if it's right or wrong) of his actions.