With general sploshing slapstick which is very popular it can be mainly tit for tat. So both / all parties get messy.

With messy humiliation I feel its generally when one person is being messed up while the other one dolling it out remains clean throughout.

So the one sided element means that one person kops it. So the scene is enhanced by the contrast one messed one clean.

So at the end of a role play one is sitting there completely messed up contemplating their fate while the one who has just dolled it out could be standing next to them perfectly clean with a grin on their faces.

the one sided element thus enhancing the role play.

I've been the evil splasher before, but for me, my plasterer needs to remain clean. It's all about me being humiliated in the smelliest condiments available x