Why would "dead flesh" burn better? I see the point of making skeletons and, in a lesser degree, zombies vulnerable to bludgeoning weapons, but fire?
Ever tried lighting a piece of beef jerky? I haven't, but it may be a nice experiment.
The description of zombies I know is human remains just starting to decompose. I don't think they are very dry, to the contrary, they are a foodstuff. Skeletons should be much drier, but I think bone burns quite poorly.
If you were to say mummies are vulnerable to fire (I know, standard) I would agree. Dry as a tinder and soaked in embalming fluids, a walking torch.
I think skeletons should even be more resistant to fire. Unless you manage to damage the bones, it doesn't to do a thing. With living creatures you get all kind of damaging effect starting with blisters and turning worse quite fast.
Just my two cents.