Is platelet rich plasma healthy for the body even if its good for the knee?

No problem. There is no problem for your body when you get a prp injection in your knee. These are just your platletts that come from your blood that are spun down in a centrifuge.
Unhealthy? Platelet rich plasma (prp) is created by spinning blood in a centrifuge and drawing off the heavier, platelet rich, component to use for medical purposes. Typically it is injected into the knee or tendon of the same person from whom it was obtained, so its as healthy as they are. Use of prp is experimental and has not been proven to help with the problems for which it is being recommended.