Here is some good information on treating splayed legs in ducklings:
https://www.metzerfarms.com/SpraddleLeg.cfm
http://www.poultrypedia.com/poultry-podiatry
I have found those incubator mats can sometimes be a tad slippery - could you try switching the duckling onto towels, and see if she gets around better? Do you still have her in the incubator?