What clothing should I bring to Mt. Titlis in Switzerland during December?

Winter mountain night temperatures are comparable with the temperature inside a freezer. Daytime temperatures are no higher than those inside a fridge but can be lower and feel lower still.

You will need serious winter clothes, meant for serious winter weather. And if you are from a warm climate you need extra layers over what locals would wear. Long underpants and a warm long sleeved undershirt at least. Thermo versions might be needed.

On top of that you need a hat, scarf, mittens (or gloves but mittens are warmer) and sun glasses. Keep your outer clothing well closed, up to your neck, unless you feel hot.
And good boots that keep your feet warm and dry.

It depends how long you are staying outside, as tourist I recommend: [I was there several time]

On top, 3 layers are often enough (shirt, pullover, winter jacket), bottom: 2 layers (you can do also with one layer [jeans], as I do it, but if you do not move, depending of winds, it can be cold, I do not recommend it).

As shoes: I recommend the moonboot: light and very effective for keeping heat. But they are just good for simple walking on snow, not for hiking.

Glove, sunglasses, and bonnet is essential.

As tourist, I do not recommend more equipment, and you can heat in restaurant. But if you want to remain outside several hours (and maybe on evening), you need technical wearing: go in a sport shop and ask. [There is too much risk to get wrong equipment if you just order stuff by "name/type"].

(according to the precision you just made)

It is difficult to answer.

If you just want to go up the mountain, look around, take a couple of pictures, you don't need the whole layers thing and technical winter equipments; you will not stay out that long.

I would have regular winter boots; I don't think you will want to venture out and hike; so no need for specialized hiking boots.

Also a regular winter coat, no need for a fancy technical and expensive winter coat if you will be using it for just a couple of hours (remember that down in the valley it will not be that cold).

Also a good hat and a pair of winter gloves; maybe a scarf if your coat does not cover your neck.

It's winter and the elevation of Titlis is 3238 meter. That means the temperatures will be far below 0° C, and it might snow. Bring winter clothes (i.e. a thick jacket, hiking boots if you have them) or ski clothes if you want to go skiing.