Can you be overprepared for the viva? I’m not sure what that would look like.
Can you waste time on activities that don’t really contribute much to your viva preparedness? Sure.
Can you focus so much on prep that you start to stress yourself out? Definitely.
Perhaps the best question to start exploring this topic is “How will you know you’re prepared?”
Set some criteria. Figure out tasks that will help meet them. Do the work.