It really varies and depends on where you live. Developmental disabilities is a narrow sub-specialty in psych. Most of what PMHNPs deal with are mood disorders, anxiety, psychotic disorders, substance use, personality d/o, and most commonly a combination of all the above.
If you live in an area that has a huge shortage, it really won't matter and you'll get a job. Having experience just gives you a bit more opportunities that's all. Also, it does help when you become a new-grad working with complex psych patients.