OK, so "I will sunbathe on your grave" probably isn't as tough a threat as "I will spit/piss/dance on your grave but you might give it a shot next time you're bitching at somebody. When these two women did it they drew a lot of ire, one guy saying the women should be buried alive.
I see no problem with it as long as they clean off any oils they might have slimed the slabs with. Ths person taking the picture might have been the creepiest person in the mess.
Thig is, I have always liked bench type gravestones & I would hope people for generations to come might sit for a spell. No dress code, profanity, smoking, drinking AOK.
I don't know if sunbathing is allowed in cemeteries in general. Cincinnati's Spring Grove Cemetery has some secluded areas currently but I don't know their rules.
Anyway, if you don't want people lying on your grave, you can always put iron spikes all over it.