Please explain "It's an absolute casserole down there"

Hi. I know what a hemorrhoid is, and I know how a "[casserole](https://images.app.goo.gl/vzCpx9h46czWW97cA)" looks I guess. But I don't get it. I understand is not a usual way to describe hemorrhoids, and... well... people don't talk as often about hemorrhoids as they would discuss weather or sports. But why is it thaaat funny? (For many Top quote from all episodes/series).

I love Mike, he is hilarious and awesome in many ways, but that quote was for me just "kinda funny" but I got more confusion than laughter.

Someone please elaborate on why is it that funny?