Honestly speaking, I don't think this is fitting_consequences or karma, just bad luck. To start with, all of the yukkuri all looked down on buri/poo poo slave so, it's not her fault that she despises retard Reimu (didn't mean it's a right thing to do). Also, all of the other yukkuri also looked down on Marisa and retard Reimu when they are in the pit, so if it is really fitting_consequences, they also have to suffer the same fate as those. Shit pit is actually a great benefit for strays as it's the only way for them to release the steam without doing anything "illegal" (can't find a better term).
Honestly speaking, I don't think this is fitting_consequences or karma, just bad luck.
It's not fitting consequences because she looks down on the buri but because it's her own fault for falling, it's not bad luck. As you can see in page 3, the proper procedure for using the shit pit is to shit on a leaf and throw it in. The fact that Maricha decided to shit directly is her own fault for being stupid. It's literally like those people who fell to their death because they take selfies in cliffedges, can't really call those bad luck can you? But shitting in it directly is how yukkuris use the shit pit in almost every other work? True, but: 1. In most of those other works, yukkuris (koyukkuris in particular) shitting directly in it also quite often results in them falling in 2. In this work, every other yukkuri shits using the leaf procedure (mentioned by Maricha herself mind you) so "Maricha didn't know any better" isn't an argument
I agree with your shit pit is beneficial for strays argument though. The poo-poo slaves that are thrown in are usually either shitheads (as punishment) or buris (who doesn't mind eating shit) so it's not like they're really doing anything bad, most of the time anyways.
Kay, I missed the leaf part, my fault, thanks for pointing it out mister Ahegao.
Yeah, it's clearly her fault, LOL. But I kinda understand why she's doing that. Shitting on a leaf and throwing it to the pit is more like a duty, an everyday task, not a punishment.