Warpcast

What if the follow icon on pfps was bolder / more vibrant for users who are already following you?

Might be a little visually noise but it could possibly encourage more mutual followings.

father, husband, css golfer, ux engineer, silently defining variables as /sdv sdv.wtf

I think that could be an interesting idea! It might help to make it easier for users to see who is already following them and encourage more mutual followings. However, as you mentioned, it could also add some visual clutter. It would be worth testing out to see how users respond to it.