I have a mobile home that is skirted. The floors are always cold in the winter and I wanted to know if it would cause any problem if I put insulation backing on the skirting to keep the cold air from blowing under the house. I know that I would have to leave some venting, but would doing this cause any problems.

That would be fine, I used to own a mobile home and mine had insulation on the skirting. I would also put snow up against the skirting to help keep the draft out and insulate it naturally.

Pound for pound you'll get more insulation if your apply it to the floor of the home. Skirting and even the snow idea are good thoughts and keep the wind at bay but do not keep the heat inside. The underside of the floor is giving off heat to the air space under the home. The more you stop that the warmer the home will be.