It might make more sense for them to linger in the more open area encounters, but ever those would biodegrade over time.
Yes they should biograde after a (not too short) while. After all, two battles can take place at Druids Grove and if there are still the corpses from the previous battle lying around it's kind of strange or the Bugbear you kill on the hill. One would like to think that even the Druids bury their enemies and give them to Mother Earth as fertilizer.
There should be changes. Waukeen's Rest should also stop burning after a certain time...
But there might also be a slight chance that the corpses remain for the Animate Dead spell, or at least their bones?