I think that would really depend on who you have running the booth, it might be a big hit kidding.
We had them at our carnival in school when I grew up, but everyone is nuts now days about PC, and you can't do anything without offending someone, so I'd stay clear of it too.
Frankly I'm not sure I can think of anyplace I would consider appropriate for a kissing booth, and I know I wouldn't support one at my child's middle school. There's just too much potential for trouble. Suppose the person in the booth doesn't want to kiss the ugly/gawky/nerdy/goth/poor/disabled/fat child? Suppose a gay student wants to be in the booth, or wants to kiss the straight student in the booth? And if you were thinking it would be for grownups, do you want to teach middle-schoolers that it's OK for mom and dad to kiss other grownups on the lips? And you still have the gay/straight issue with grownups -- and maybe the unwillingness to kiss the ugly/gawky/nerdy/goth/poor/disabled/fat grownups, too! Others may have other opinions, but I wouldn't go near this idea myself.
Do you think a kissing booth is appropriate for a middle school carnival? I tend to be a bit cautious when it comes to things like that, but don't want to throw my concerns out there if it's just me.