No, not at all.
Usermin is a completely separate (optional) tool, unrelated to Virtualmin, though the Virtualmin installer installs Usermin because it is a webmail client, among other things, that integrates nicely with Virtualmin and Webmin.
Administrators and domain owners login to the same UI. When you login to Webmin (Virtualmin) as a domain owner user, that is not Usermin. Usermin is a webmail client, among other things, that runs on port 20000, by default.
The access domain owners have is configurable in Virtualmin, in Server Templates, Account Plans, and Virtualmin Configuration. You can grant Virtualmin domain owners a lot or a little access. It defaults to a little (though it could be even less, the default is intended to be a usable subset).