I have the same thing exactly. You could pull the plastic A-pillar piece off and use a heat gun to warm the plastic and bend the corner up a bit so it curves towards the headliner more
I managed to work a piece of duct tape up behind both pieces to hold them in place. Using a small flat tool like a small spatula to press it on is more effective than trying to squeeze fat fingers in there.
From what I’ve seen on the cars I work on, either the headliner isn’t allowing the pillar to seat fully, or the pillar itself isn’t seating because of the something underneath like the attachment hardware. I’m not sure how the mustang pillars are secured, but I’ve replaced pillars on Touareg’s and Tiguan’s due to poor fitment because of the one-time-use crash clips they use