Road cycling is definitely more aerobic than skiing. But mountain biking has a lot of similarities with skiing. It's more anaerobic, works on balance and agility better, works on sighting lines and avoiding obsticals as well as terrain changes. There's also the skills dealing with pitches and braking, and when to lay off the brakes, which transfers over to skiing well.
I don't get a chance to mountain bike nearly as much as I'd like to, but I bike commute. Last week I didn't ride because I was in VT skiing all week, and I wasn't able to ride the week prior, or most of the week prior to that. My commute this morning, just plain sucked. Yeah, the 20 degree temp had a little to do with it too, but even though my legs are strong, my aerobic fitness was in the toilet, and my average speed on the way to work showed it pretty drastically. Hopefully, the ride home will be a bit better.