Sure his atrocities, and the rumours about rumours regarding his heritage are fresh in our memories, but Aegon the Conqueror killed thousands in his conquest, and was married to his sisters, and he is remembered as a great king, because of his overall accomplishment of conquering most of Westeroes. So if we forget all the minor torture and terrible acts he committed, we see that during his tenure as king, he quelled several rebellions as a boy, before he was tragically assassinated at his own wedding, but is this enough to make him a good king?
History really seems to disagree with your opinion of King Joffrey. Joffrey was mostly a figurehead and had little to do with actually squashing the rebellions. In addition, though unknown at the time, historians know believe that Tyrion Lannister was actually hugely influential in the Battle of Blackwater and that his claims of "saving the city" were not as empty as believed for many years.