The criterion for a "bi-polar" diagnosis are outlined in the 5th edition of the "DSM" (Directory of Statistical and Mental Disorders) , the bible of psychiatrists in the United States and many western countries. The evaluation includes both verbal responses during interviews and observation of inappropriate behavior. Clearly a person with diminished function can only be evaluated with difficulty. Those with diminished intellect often perform inappropriate actions. The criterion are simply not designed for the handicapped or those with brain damage. That is not to say that a psychiatrist may decide to provide that label on a patient and treat them for the disorder. This usually involves medications, which may further degrade mental function, causing loss in quality of life.
A person with a developmental disability may have a psychiatric disability as well as a dual diagnosis. I agree it can be difficult to distinguish the two and there are specific clinical guidelines to the use of psychiatric medications in people with developmental disabilities. It would be essential to see an appropriate specialist in the field if there are concerns and if the treating specialist suggests that medication may be needed to discuss with them why, how they believe it may help and the potential short term and long term side effects. With any medication you can find the full information in the P.D.R. (Physician's Desk Reference) and the medication website itself which can then be discussed with the treating specialist.
Yes, many people with developmental disabilities have co-occurring mental health issues.