Commits obviously have an author associated with them; but is it possible to find out which user created a tag or a branch on a CVS repository?
If you have history enabled (which I think is the default) and your tag was created using cvs rtag then you can use:
cvs history -a -T
If you want tags that are created from within the branch via cvs tag then you are out of luck as the history command does not support that.
One thing to bear in mind is that in CVS tags are not repository-wide. They apply at the file level. So it is entirely possible for different people to apply the same tag to different files at different times.