In 1990, my parents changed their matrimonial regime to adopt the universal community with full attribution clause to the surviving spouse. However it seems that this change has not been approved by the court [...]

[...] My mother just died. What is the consequence of this oversight?

In this case, it is not possible to liquidate the matrimonial property scheme of your parents on the basis of universal community. Indeed, the Civil Code provides that "the approved change effect between the parties from the date of the judgment and, in respect of the third, three months after a notice to have been placed next to one another and copy of the marriage certificate.
This implies that to be valid, the adoption of a new matrimonial regime must be approved by the judge.

Warning: the approval procedure has been significantly changed by Act No. 2006-728 of 23 June 2006 on the reform of inheritance and gifts. The approval of the matrimonial regime change is not systematic