To be honest it is really hard to give you a definite answer. Over the years I’ve read so much stuff on the internet say this many different things. However, I can say 100% that what might be bad for one person might work really well for another.
Myself I have not used a single foaming cleanser that has helped with my seborrheic dermatitis. But shaving foam never irritated it and shaving actually improved it for me. Also things that contained oil seemed to be bad overall. Yet currently I’ve been using a lotion that contains sunflower oil for over 7 months with amazing success. You can see the details of my current approach here.
When I tried a vitamin C serum, it was not very effective and actually irritated my seborrheic dermatitis. However, I’ve read one person who had amazing results with it. One thing I did notice is that supplementing with vitamin c does help to improve skin tone and keep inflammation down.
Hope that helps. Let me know if you need more information.