Side Note: This question was specifically about setting the default username in an imported instance. However, for completeness, you can also set the default username for a distro that was installed from the Store (or wsl --install) with:
<distro>.exe config --default-user <username>
For instance, if you installed "Ubuntu 20.04" from the Store, you would use:
ubuntu2004.exe config --default-user <username>
The .exe here is an "App Execution Alias" in Windows. You can check the name by going to "Manage app execution aliases" in the Windows System Settings.