I remember that we had mistletoe when I was little. You do not see it much any more. The idea is a classic and unfortunately is a long lost tradition. I think it is sweet and would be a fun reminder of old traditions. We have not done it, and I don't even know where I would find any mistletoe, but I still think it is a classic traditon.
The tradition of kissing under the mistletoe is not an outdated notion. Traditions are meant to be passed down through the generations, and kissing your crush under the mistletoe is a funny activity. Moreover, if you get stuck under the mistletoe with someone you do not want to kiss, you do not actually have to kiss them. Let people have their fun and their traditions if it is not hurting anybody.
Generally speaking, all religious traditions are "outdated" to some degree. But, I think that sort of reasoning is begging the question. The purpose of a tradition is to not change throughout time, so in that sense, there is no way for a trandition to really become "outdated." That said, I don't think someone should be entitled to a kiss based on being in proximity to anything.
I don't think that the tradition of kissing under the mistletoe is an outdated notion. It is a sweet tradition that makes people happy. Yes, it is an old tradition, but just because something is old, it doesn't mean it is outdated. There are many other traditions that I would find more outdated than kissing under the mistletoe.