The Stipa-Caproni was certainly odd, but I wouldn't call it ugly. Imho, the ugliest plane ever made was the M-15 Belphegor, which has the unusual distinction of being a jet powered crop dusting biplane.
I once saw an unhappy looking AEW Nimrod sitting near the edge of RAF Kinloss (as it was at the time) - pictures don't really do it justice as to how ugly it was.