Automatically make Mobile Accounts Admins

That command won't work. The -n flag expects to get a username for an account that exists in Active Directory, not a command like what you have there. Unless that was supposed to be a second command on it's own line. Even if it was, you need to specify a username with -n when using the createmobileaccount

As for then making that account a local admin, you will want to use the dseditgroup command. dscl shouldn't really be used any more for changing admin status and even group memberships on accounts. It's still useful for other things, but I would not use it anymore for changing admin status.

Example with dseditgroup:

dseditgroup -o edit -a <username> admin

The above requires root or sudo to work, but if it's done from a script running in a policy, that's taken care of.

You can check the admin status of the account after the command above is run with:

dseditgroup -o checkmember -m <username> admin