It can be tweaked slightly. Actually username flagging is generally quite helpful, especially in conjunction with other flags. Sometimes a common username like this can give a false positive. Though if it was lumped in with IP and/or email flags it's more reliable.
Personally I would leave it as it is.
But in Admin CP > Options > Spam Management you can change the number of flags required to moderate a user. If you change it to 2 it would need e.g. username and email or email and IP or username and IP before moderating the registration.