I have not got the Zombie kisser, or the kissed by a zombie, achievement yet. I know I have kissed others while wearing a zombie skin. Is there some trick I am missing or have I encountered a bug?

they don't show as retired to me, but maybe they are but aren't marked?. There might be a cache delay though. So stick with your mature skin for a few hours and see if that matters. I get kissed by zombified folks a lot in kissing threads, but it may matter if they're mature, so I dunno.