NTUSER.dat is basically the user's copy of the registry. It **might** still copy fine if you load the CMD as an administrator. A better alternative is to use the /XF switch (in robocopy) to exclude that file. It'll only really be useful on that specific computer anyway. So (assuming you're interested in backing up important files rather than user settings/environment), there's no real reason to back it up anyway.
You could also forget trying to mirror the top level of the user's folder, and create separate lines in your batch job for the particular folders you want to keep (Documents, Downloads, Music, Pictures, etc). In the one batch file, you can just copy-paste your current robocopy task multiple times for each subfolder. It'll just work through them one at a time.
For example:
ROBOCOPY "C:\Users\UserName\documents" "C:\Backup\UserName\documents" /mir
ROBOCOPY "C:\Users\UserName\downloads" "C:\Backup\UserName\downloads" /mir
etc