I am a physics major at UCLA with a busy schedule. I am currently taking 3 upper division physics courses every quarter while balancing 10-20 hours of research per week. I would like to learn more computer science, so I wanted to take a couple of CS classes for a Pass/No Pass grade without stressing about my GPA. Would Grad Schools frown upon the idea of taking CS classes for P/NP grades? I feel like my reason is justified, and the CS courses are not a part of my major, but they could potentially relate to it as a needed skill.
I don't think you have anything to worry about. You seem to be doing the right thing in your major and that will be the important consideration for grad school. But anything you do to increase supplementary skills is a positive thing, not negative. For example, even an online no-credit course can increase your skills and would have a small positive effect. If you were doing this in physics courses it would be viewed differently.
You have a good explanation to give for your choices. If asked, just present that. No one should think less of you or your abilities.