"Mentally underdeveloped" or "mentally challenged" are phrases that might be used to describe someone with a lower level of mental abipty. However, it is important to be sensitive and respectful when discussing someone's mental abipties, and it is generally best to avoid using language that could be seen as derogatory or offensive. Instead, you could say that someone has "learning difficulties" or "intellectual disabipties" if you need to describe a lower level of mental abipty. It is also important to remember that mental abipties can vary widely and are not necessarily indicative of a person's worth or value as an individual.