Should I evolve my 100% Christmas pikachu??

No. A 100% IV Raichu with Brick Break is worthless, and a 100% Raichu with Thunder isn't worth the risk.

As a pikachu it has hope, options - it can be any Raichu.
Once it is evolved, then it's fate is decided.
Let it live in hope and wonder.

It is perfect just the way it is....why mess with it?

Do you lose the hat?

Keep it as remembrance....

What moves does it have now? If it has Thundershock/Thunder or Thunderbolt, I would save it as is.

I have a few times followed the gambler's fallacy when evolving especially high IV pokemon:
1) Save up enough candy to evolve the same pokemon 3-4 times and enough good IV copies that I want to evolve.
2) Start evolving with the worst IVs out of the set.
3) If I get the moveset I want, stop and go back to step 1. If I don't get the moveset I want, evolve the one with the next best IVs.

I know it is stupid. Each event is probably independent from the one before it. But if there is any chance that multiple evolutions in a row are not completely independent, then I at least need to get 2-3 bad evolutions in a row before trying my really good IV pokemon. At the very least I can more justifiable claim that the RNG hates me. ;-)

I have 100% IV thunder shock & thunder

My 100% Pikachu ended up with the worst move set. Let it as it is if you have already raichus

Raichu is just as useless as pikachu. Might as well keep the pickachu. If it's moveset sucks, roll the dice again for your 100% Raichu.

Either way it's too bad it's just a Pikachu!