The court is legally bound what is in the best interest of the child within certain legal (including constitutional) limits. It's not anything in the interest of the child, no matter what. That goes without saying. A question here is whether the court may legally count a parent's ideological rituals when it comes to assessing the best interest of the child.
It isn't a ritual, it is an "expression" of her belief. Whether it bothers the child is entirely unknown. Based on how long this has lasted, it appears to be parents arguing and bickering. Otherwise, the woman would have gotten rid of the darn thing. I think she is doing it just to piss off the father. If she was doing it to harm the child, I'd imagine the child's legal rep would have raised the issue.
Why James Madison would choose a NY Post article about an appellate case of two parents squabbling over a child's custody, when the appellate justices note the clear right for the mother to believe what she wants to believe and only indicate that the expression of the belief (as its direct effect on the child) should be taken into account when determining the suitability of her as providing a home for the child, is really a mystery. JM isn't usually tabloid driven.