This is an unusual behavior for any child and one must first consider why the child is acting this way.
If the child is very young [9 or under] then it may be simply a way for the child to relieve guilt over a wrong doing. However it would be wise to try and find out how the child got the idea for a nude spanking in the first place.
At 10 and older one must consider the possible sexual aspects of the request. Many children and teens find sexual stimulation through spanking and nudity can add to that stimulation for some.
In the end it is up to the parent to judge whether the child is just doing what they believe will relieve their guilt, or whether there is an alternative reason the child is behaving in this fashion.
One way to help determine which is the case is to fulfill the child’s request once and if the child makes the same request again within a few weeks then there is most likely an alternate reason for wanting the spanking.