As of RouterOS 7.6 there is no official command to create a directory on a RouterOS filesystem. However, there’s a trick involving a SMB share. By creating the SMB share, RouterOS will create the directory. After that, you can delete the SMB share.
The following script will create the backups directory:
/ip smb shares add name=deleteme directory=backups ; /ip smb shares remove [find name=deleteme]')