Both worlds are basically our world, only populated with a hugely varied collection of freakish monsters; the ordinary names, clothing, jobs, behaviour, architecture, social structure etc. contrasting weirdly with the bizarre creatures. And it's worth noting that in Ugly Hill
, all one-eyed creatures (as varied as they may be) are treated as a single race, an oppressed minority at that, while in Monsters, Inc..
the two one-eyed characters — Mike and Celia — have menial, probably low-paying jobs, and are dating each other.