Goes nuts trying to find the perfect tree: Asriel! As Flowey, he may even try to cut down the tree himself.
Goes overboard with the decorations: They're neutral when it comes to decorating, Papyrus and Undyne have all the hype for everybody. XD
Messes up the kitchen making cookies and other sweets: Frisk! May partially be Undyne's fault. :P
Has to save the other from being suckered in by a kid faking a Christmas Shoes-esque sob story to get some cash: Implying Flowey isn't that very kid. :P Frisk isn't fooled and is just -_-
Panics over buying the other the perfect gift: Asriel again. ;;
Insists on ugly sweaters: Frisk, hands down.
Insists on all the usual Christmas specials, even the terrible ones: Frisk doesn't care what they watch, Alphys will likely pick (or Undyne will want to marathon everything)
Insists on going caroling: Frisk, but really, it depends on what everyone else is doing.
Leaves cookies and milk for Santa: Frisk. Don't forget the butterscotch cinnamon pie! Hugs for Santa Asgore too. ;;
Tries to put Santa hats on the pets: Frisk because aww.
Has the relative that sends them enough fruitcakes that they're running out of room in the closet: They live with Toriel, so they get the most delicious Giftmas food on earth~