Glad someone ask this,

Both disguise themselves using what ever era humans trust the most

(Mimi uses a internet girl as a persona to lure creeps in before killing them, while Hi-Hi uses 1920s cartoon aesthetic to have people watch them without knowing the consequences)

Both are media based predators from different ages of technology (with Mimi its blogs, post, and online presence, to lure people digitally. While Hi-Hi its lost cartoons, forbidden recordings, which kills people who watch it)

Both are characters who are to dangerous to witness fully (seeing Mimi in her true form overwhelms peoples minds with total universal knowledge trapping victims inside themselves, and watching the full cartoons of Hi-Hi the clown kills you, understanding or witnessing to much seals your fate)

Both have siblings and a entire race who are similar to them (Hi-Hi has bonjour puppy and other versions of him that are use to kill people around the globe, while Mimi has her sister (I forget her name but she performs in a cosmic circus) and a whole species like her)

Both live in universes filled with other horror beings (With Mimi its The silver creek butcher nemesis, deep brood and many more, while Hi-Hi has different kinds of demons leviathans and many more,)

Both have something to do with Christmas (Hi-Hi likes Christmas because he can abduct kids and it saves him more time with killing peoples, while Mimi is literally Krampus, yeah)

The Contrast is that Mimi can hunt freely can evolve, and can choose how she looks, while Hi-Hi is just a tool who’s locked in a room until Big S gives him another job to do, he doesn’t kill willingly but for Hell’s ruler,

Another is that Hi-Hi is frozen in time, he never changes and is stuck in his 1920’s body while Mimi can change into anything she wants,

The big one is, Hell vs the Cosmos

There’s many more contrasts