You could use the login form widget (https://appstore.home.mendix.com/link/app/87/Mendix/Login-form). There is a Casehandling tab which allows you to specify whether you want to convert username to lowercase, uppercase or do nothing.
Make sure all usernames are stored in lower case and change the login.js script in your theme to change any input in the username field to lower case before submitting to the server should be enough.
Lets try this one ! Turn off the autocapitalize option in login.js file. It will work.