Your Action is expecting string parameters, but you're sending a composite object.
You need to create an object that matches what you're sending.
public class Data
{
public string username { get;set; }
public string password { get;set; }
}
public JsonResult Login(Data data)
{
}
EDIT
In addition, toStringify() is probably not what you want here. Just send the object itself.
data: data,