Huxley is not trying to mock religion on the savage reservation. Instead, he is illustrating a theme of the book: that a trade-off exists between the sterile and soulless but clean, bright, orderly and superficially happy society of Brave New World and one that allows freedom, intellect, passion, poetry and individuality to flourish.
On the savage reservation, passion and religion still exist. We see, however, that this comes at a price. For example, the society is dirty. On the way to the religious ceremony, Lenina and Bernard pass a dead dog thrown on a trash heap, people have lice and the savages don't bathe regularly. Linda is bloated and ugly, and we see people who are visibly old and diseased. Religion allows the beauty of deep, intense feeling but also involves suffering: a young initiate is whipped as part of the religious ceremony Lenina and Bernard witness and blood runs down the young man's back. Huxley shows this not to mock religion, but to show that being fully human, which includes worshipping God, carries a cost. In a society with disease and ugliness, people need to learn how to bear suffering.
At the end of the book, Mustapha Mond makes an argument in favor of his well-ordered, safe world, and the Savage responds with an impassioned plea for religion:
But God’s the reason for everything noble and fine and heroic. If you had a God ...
"My dear young friend,” said Mustapha Mond, “civilization has absolutely no need of nobility or heroism."
What Huxley mocks is not religion, but Mond's vision of a society without nobility, heroism or God.