Yeah, "Hot" is just short for Hot Garbage. It's basically "New" for posts with a couple of upvotes and maybe 1 post.
I think it's a divide by zero thing where posts are weighted by activity divided by age. So a tiny amount of activity on a new post is weighted the same or more than an active post that's a few hours old.
Edit to add: Top 6 Hour seems to be where it's at for a mix of fresh content that is also active.
Hey there! Mlem doesn't control the way that posts are sorted when using the "Hot" sort mode - that's something that Lemmy does. The mechanism of the "Hot" sorting algorithm is described in detail here.
You may prefer to use one of the "Top of..." sort modes instead, if you'd like to see more popular posts. In Mlem, you can change the default sort mode under Settings -> Sorting -> Posts.