Is there any kind of psychotherapy that can help improve mental retardation?

Not exactly. Mental retardation does not change but social skills and ability to cope in the world with "slowed learning" could improve through behavioral modification .
Many help function. The goal of therapies is to help a person with intellectual disability learn to function in the community as independently as possible. They address behavioral, occupational, physical, academic, communication and social skills. Funding for therapies may be available through ssi and medicaid. A free and appropriate education in the least restrictive setting is mandated by the federal law idea.
Improve relatedness. This is a poorly researched area. From what's available, individual and group psychotherapies can be used for emotional and behavioral issues that arise, because these people often have dual diagnoses (mr plus depression, etc). This is not really to improve the retardation itself, but it can improve relatedness and other aspects of living. Supportive techniques can be very useful.