Do I need an admin username and password for Genymotion?

Yes for some Genymotion PaaS devices, but not for Genymotion Desktop account sign-in. On PaaS, the default device login is usually genymotion, with the password depending on the cloud provider or instance settings, and you can change or disable it on some platforms. See Genymotion PaaS Overview and I have just changed my password, but Genymotion Desktop still says "bad username and/or password".

For Genymotion Desktop, you sign in with your Genymotion account email and password, not an admin username. If you created your account with SSO, you must set a password first because Desktop does not support SSO sign-in yet.