ASCOM specifies that a driver should not open user interface, with the exception of the setup dialog. This is because it could be running on a remote PC where the only interaction is through ASCOM. Some drivers will show a dialog but it must be additional and never need the user to interact locally. For example showing a message box is not allowed, not even if there is an error.
Simulators are a bit different, the UI they provide is really a way of providing an alternative to a hardware connection, and not all simulators do this.