Honestly, I agree with Klaus that the PHB cover was by the weakest of the three, when as he says, it should very much have been the strongest. Any new cover seems likely to be better, based on the generally higher quality of most of WAR's work.
As for inappropriate, well, it's vague, but I would think it meant "unappealling" (the Tiefling was certainly "unappealling" to me, just plain unattractive, really, ugly robe, ugly horns, weak look) as opposed to "Too satanic!". Especially given the bloody great demon on the MM. It's not like religious nuts who hate D&D are capable of distinguishing between the PHB and MM now, is it?