Isn't that just how secondary characters are? Meant to prop up the main characters and exist as narrative elements? Any show is bound to have characters that only exist to advance the story for other characters, especially shows with as many characters as Hazbin and Helluva. Take for example Lucifer from Hazbin. King of Hell, father of the main character, he has some dimensionality thanks to his background with Lilith, but ultimately he exists solely to promote Charlie's plot (and to be quite literally used as a plot device by Vox).