I think there are two issues with the cake smash, and they are being squished into one and muddying the waters so to speak.
One issue is of consent. To witness a cake smash when there is not equal and total consent, is an act of aggression. And a terrible terrible thing to bear witness to as a guest.
The other issue is of appropriateness. That one is a more fluid line as to what crosses the line - some people think the line lays between no smashing whatsoever and any smashing whatsoever, others think there is a degree of smash that is acceptable.
For me that line is based on the first issue (aggression level) but also an intimacy/casual level of the event. I think what a couple can happily do in private or in a very casual situation (at home, at a picnic, at a beer-out-of-red-Solo-cups-party) is not necessarily ok to do at a formal wedding. That level of intimacy/casualness isn't limited to cakesmashing either, although it includes it. It also includes the type of kissing and PDA a couple engages in, the type of language used, the level of drunk one gets, the appropriateness of actions like licking ones fingers or using the sides of their fork to cut their food instead of a knife, etc.
So if someone is having a more casual, more beer-out-of-red-Solo-cups-wedding, a wear jeans and flipflops wedding, a back yard finger-licking BBQ wedding, well then so long as the first issue isn't an issue (both B&G agree on the level of smash) then ok, smash away... and maybe witness a groom pinch is brides butt, or an open mouth kiss, or shots or keg stands being done. But I think once you start getting into ballrooms and gowns and tiaras and tuxes and limos, cakesmashing is behavior at odds with the event being held.
And I think if the wedding is somewhere between the super casual event and super formal event, well thats where that fluid line - always first and foremost based on consent - starts moving.