Jormungandr was previously mentioned in the episode Last Christmas! (2018), with "keeping the world-eating serpent Jormungandr at bay" being one of the responsibilities that Scrooge described from throughout the year.
Scrooge and company enter Valhalla by flying through a rainbow. This is a reference to Bifrost, the "Rainbow Bridge" used by the gods of the Norse pantheon.
The character Hecka is named after the character Hela from Norse Mythology. Also these names are based on the words "heck" and "hell", which notably the former is often used in place of the latter in different forms of entertainment for censorship purposes, due to Hell being a very inappropriate subject, especially for children's entertainment.
Dewey's ChamPopular Outfit and Persona is based on his "popular high school jock" outfit that he wore in "A Nightmare on Kilomotor Hill".
In "Quack Pack", one of Dewey's weaknesses was revealed to be disapproval in a way that causes him to comically lose his strength. In this episode it's made use of to face off against Jormungandr.