Is Deja Dup compatible with Dropbox?

Yes and no.

There is nothing in Déjà Dup which makes it specifically aware of Dropbox. On the other hand, Déjà Dup can be pointed to a local path, so there is always the option of running your backups towards a path within the Dropbox folder.

As always will should test that you actually can restore, preferably from another computer connected to the same Dropbox, in case the Dropbox sync somehow screws up the backup directory. Not that it should, but you should test and verify nevertheless.