Mitigating Sera and Iron Bull'a disapproval after In Hushed Whispers?

There is still plenty of game to play and approval to gain, don't worry about it.

I agree with above, if you want to raise their approval then take them on a dragon hunt. Both get really worked up in a good way by this, but particularly Bull. As for Sera, there are going to be much more significant factors that are going to affect if she likes you or not and if your actions mean she doesn't like you, then just live with it. You can't please everyone all of the time and I found Sera one of the more tricky ones to keep on side and remain true to yourself, particularly if your PC is not willing to humour her as was the case with my first Inquisitor.

The mage/Templar decision is really early days and there will be a number of other decisions where you will find that one or more of your companions may not approve and yet others will either approve or greatly approve.

bull is dragons, though you can still net it back in other ways, a bit harder with him if you don't kill a dragon but do-able. as for sera, it's easy, she have prompt dialogue after every main quest. also doing the war table missions she gives you net approval.