Is it harder to mount tires when they're cold? It's about 12 celsius in my workroom, and I'm trying (and failing) to mount my tubless tires (Hutchinson Atom) on my ultegra wheelset. I've tried soapy water to lubricate, pushing the tire beads into the rim's center channel, and even using levers (a nono, I know) but I can't get the tire to mount. Can I blame the cool temperature? Or is this failing entirely my own?

FWIW I'm able to mount my Schwalbe Durano's on the wheel in a minute or so.

Tires intended to run tubeless are often a tighter fit than regular tires. That's the main reason why you're struggling.
And while the rubber being generally stiffer aint helping, the wire bead doesn't change characteristics to any noticeable amount at anything even vaguely resembling room temperature.

I have noticed that it harder to change tires with cold HANDS.

Put the tire in the oven for a while.

The rubber and your fingers are both stiffer, so yes it's more difficult. Get a better heater in that work room!