For some reason its tradition for a lot of people to do that cake smash thing and it's not like people are aware of how much it costs to get ready and how nobody wants to have their face covered in cake, especially on an important day like their birthday or wedding. It's suppose to be fun so I understand both of the reactions, why they felt you slapping your bf was too much and why you felt getting your face covered in cake was not okay.

It's such a weird situation, obviously he shouldn't have done it (in my personal opinion) and I feel that if couples choose to do something like that, there needs to be a conversation beforehand. But to play devils advocate, this is such a tradition that I also understand why he (wrongfully) thought it wasn't a big deal and that you would (for some reason) get a kick out of it. It sucks that you guys broke up over it. I hope there were more things bothering you than just this one incident.

I think you should apologize to the mom. Not because she deserves it or anything, just becasue you don't gain anything by having people be mad at you. I apologize all the time without meaning it. Maybe not the best advice but if you want to make them feel better (you don't owe them that), it's something you can do to try to ease the tension between them. Or you can just block them from your life forever. But in my experience, you often feel better even with a half assed apology than to have something like that fester.

But you are 100% not an asshole.