This is a running theme in season 3 (and in the series as a whole): The family is becoming jaded by Rick's shenanigans. Remember how Morty was frightened and hysterical when Rick told him he's going to nuke the world in the pilot? Well, now he's nonchalantly defusing the Neutrino bomb that Rick has built in Vindicators 3, commenting angrily that this happened way too many times. And the Summer that was scarred by her experience with Rick's ship in The Ricks Must Be Crazy? She's willingly joining his adventures and unflinchingly murders people in Rickmancing the Stone.
Morty and Summer were startled by their grandpa's nudity in the past (in M. Night Shaym-Aliens! and Big Trouble In Little Sanchez, respectively), but this has happened so many times that they're simply accustomed to it by now. Note that even Beth doesn't really seem to be bothered by it when she returns to the house in Rest and Ricklaxation, and barely even cares that the house is in shambles. Rick's misadventures are gradually turning his family indifferent to all the strange occurences around them - and shaping them to be more and more like him.