I will always be an advocate for new grad nurses getting experience before going back to school. I have seen and worked with lots of NPs who went straight to grad school after undergrad, and they just do not have a grasp on what the real-world healthcare system is like. However, my experience comes from ICU & ER, where you need to be extremely clinically competent to be able to do your job. I understand that PMHNP is very different because clinical skills are not an important factor. However, you'll be hard pressed to find any high-quality DNP program that will even consider your application with no real nursing experience. You can absolutely find lower-quality DNP programs who will take anyone into their program because all they care about is $$$. But then you will graduate 4 years later, with no real nursing experience and your resume is not going to even be comparable to new NPs who had 5-10 years of nursing experience before getting their PMHNP.
Of course, it is your life and your career. If you know you want to get your PMHNP, I would try to get experience in an ER that has a high census of psychiatric patients. That way, you are getting experience with the psychiatric population but you also get tons of exposure with regular medical patients, traumas, pediatrics, etc. At the end of the day, people have TONS of medical issues. And a lot of psychiatric patients you will see will also have lots of co-morbidities. Its invaluable to have experience with a wide-range of diseases, different patient populations, etc. And it will set you ahead of the curve in your PMHNP classes. A lot of people on these threads tell people to get true psychiatric experience on a psych unit; obviously, this makes a lot of sense. But I've found it incredibly beneficial to work in an ER where I'm not exclusively seeing psych patients so I can understand how issues like DM, CHF, asthma, etc impacts psychiatric care & medication administration. I would say about 50% of the psych patients I see in the ER have medical co-morbidities.
I could go on and on about this. But at the end of the day, you have to make the decision for your career. I believe you will be a much more effective, compassionate, successful PMHNP if you get just 2 years of nursing experience before pursuing DNP.