That's always been odd to me. Same with parents kissing their kids on the lips.
Oh god, my mom does that to me and my brothers!
And we are all grown adults at this point: 29, 35, and 37.
Look, it's odd to me too, but I rarely see her anyway, so I just go along with it to make her feel loved and happy. The quick peck on the lips only happens when we are saying goodbye. Which happens maybe once, maybe twice a year? And she isn't doing anything creepy or unwholesome...so we all let it fly. But it's still odd. Lol.