There are a couple reasons why this would work.
Lore continuity: veigar in the lore wants to be an evil overlord because of all the time he was tortured by mordekasier. Yordles seem take on the traits of their environment, and veigar took on the evil trait of mordekaiser. We already have a bad Santa veigar, so bad Santa mordekaiser would give a reason for bad Santa veigar to want to be evil and ruin Christmas.
Thematic elements: mordekaiser is the lord of death and he is building his kingdom in the death realm with the spirits of the dead. So what if mordekaiser wanted to take the CHRISTMAS SPIRIT away from everyone? What kind of evil could he make with that spirit? Good lore for a skin, right?
abilities that can be Christmas themed:
His ultimate brings people into a dark and evil winter wonderland! His E summons a gust of wind and snow instead of a hand! His passive could look like a blizzard! ITS PERFECT!
Appearance: mordekaiser has a thin cape. But Santa doesn’t wear a cape! So instead of the cape, mordekaiser wears one of those super long Santa hats! As for the two spikes on the helmet, it could easily be made into reindeer antlers. Of course he would wear a standard Santa suit. Maybe two wreaths on his shoulders to look like his pauldrons. The mace? Well I have a couple ideas. It can be a giant candy cane, a big bag of coal with new attack animations, or a sleigh on a stick. For his W affects, I’m thinking messy Christmas lights tangled in his antlers and around his suit. When he activates his W, all the lights turn on instead of a weird green glow around him.
MordeKaiser is overdue on skins: his last skin was from 4 years ago. PLEASE RIOT! BAD SANTA MORDEKAISER. LEGENDARY SKIN I WILL BUY IT PLEASE RIOT! Edit: it has come to my mind that the mordekaiser rework required riot to remake all of his skins. But still, It is no new THEME for a skin in 4 years
Edit: the poll is meant to say “do you want bad Santa mordekaiser” but auto correct is a bitch. And I forgot the bad Santa part...