Is the mother's saying, "Would that there were no Fajr (dawn) prayer," considered a rejection of a Sharia ruling or a dislike for what Allah has revealed?

The intent of the mother's words cannot be determined without knowing the context. However, it is inconceivable that she meant to deny the obligation of Fajr prayer, as a Muslim does not become an infidel by disliking some rulings of the Sharia, naturally, though it is better to strive to make oneself love them.