I dunno. I always thought it was nasty and passive-aggressive. I found this:
From Business Insider--
Not everybody is into the idea of smearing frosting all over their partner's face, but its silly nature has kept this tradition going! Turns out that messy food traditions have ben in place for years.

For example, in Yorkshire, the bride would eat a small piece of bride's cake (which was more of a pastry than the cakes we think of today). Then, she'd throw the rest of the cake over the head to ensure a life of wanting nothing (and to ensure a huge mess to clean up). In even older times, the groom would break bread over the bride's head and guests would pick up the crumbs.

It's not totally clear how we got from picking up crumbs and throwing cake to smearing frosting across someone's face, but one thing's for sure - brides and grooms have always played with their food!

Gert, here's my take, bear with me, I'll tell you a little story: One time during a trip to Maui, my (now ex) wife and I were on a sightseeing tour circumnavigating the island. At many points, the road is quite narrow and there are turnouts to let cars pass, and sometimes view remarkable natural phenomena. We stopped at one such turnout and I noted several piles of delicately and seemingly precisely stacked rocks. Thinking perhaps there was some deep religious or perhaps native significance to these small tributes, I asked the driver about them; he quite matter-of-factly stated that it was likely a case of "monkey see, monkey do." I think it's pretty much the same with the wedding cake face smearing. Seems, if not plain stupid, at a minimum repugnant, to me. I have no idea where or why it originally started, and the only explanation I can come up with is that it flies in the face of convention in an otherwise ultra-conventional setting.

Nope...I never understood that custom, and I would have been furious if it had happened to me.