I had front end PPF installed and then ceramic coating over the whole car including the PPF. If anything it acts as a double barrier and I don't believe PPF has the same hydrophobic effect as a ceramic coating has. So in my opinion (and the detailers that I spoke to), yes it can be a good idea to put a ceramic coating over PPF.
Below is the product that was used on my car. I'm not selling it as the best out there, I'm simply posting it's claimed benefits.
Krypton 5 is a permanent ceramic paint coating that features a High Gloss, which is Scratch Resistant, Chemical Resistant, UV Resistant, Thermal Resistant and Anti-Graffiti.
Krypton 5 has a Super Hydrophobic and Anti-Graffiti effect combined mean the 5H coated surface will stay cleaner for longer, as dirt and grime will not stick to the surface and the hydrophobic effect of the coating will cause water to bead up and roll of the surface with any dirt and grime, the hard ceramic film also offers superior protection from damaging contamination and harsh chemicals.
Krypton 5 forms a permanent bond to the paint work and will not wash away or break down, can only be removed by abrasion making it a highly durable protective coating to protect your paint work for damaging contaminants.
The unique formulation of Krypton 5 has enabled it to be multi-layered which means the thickness of the coating can be increased with additional layers allowing a thicker / harder film that will increase its scratch resistance. This can be an additional service at a fraction of the original cost after the detail has been carried out.