If both people who are standing under the mistletoe are comfortable and consenting to the kiss, I see no problem with mistletoe being the propelling reason the two of them do so. However, mistletoe certainly shouldn't be used as an excuse to try to kiss someone who does not want to.
I think kissing under the mistletoe is a fun tradition, and I don't think it has to be a big deal. If you are standing under it with someone you don't know or don't want to kiss, you can kiss them on the cheek or just a quick peck on the lips. It doesn't have to be a long, intense kiss!
"Kiss me under the mistletoe" is an age-old Christmas saying that is often associated with loving couples. However, simply because you are standing under a piece of mistletoe with someone does not make it appropriate to kiss them, especially if they have not expressed any interest in doing so. In addition, no one should feel pressured to share a kiss with someone simply because they are standing under a plant.
No. Stop. Think before you lean in and invade someone's personal space for a quick one because 'tis the season.' There are actually people out there that don't care for this ongoing and debatable practice. Anyone been at a Christmas party when someone's been hitting the eggnog a bit much and they want to share the cheer, perhaps a tad obnoxiously? For that example alone. Please, just don't.