I seriously doubt that Trump came up with the "Happy Birthday Mary" idea on his own, I'm willing to bet that he has Catholic staffers who are able to prompt him on things that Catholics would find inspiring. That said, it's his call as to whether to sign off on it or not, and the fact that he agreed to it speaks volumes. I don't think he is in the least bit opposed to Catholicism. Put it this way, try to imagine Kamala hαɾɾιs making posts such as this.
Prayers for his conversion would be in order.