No, they’re not getting hit with C&D orders because that are not using any Nintendo IP. Knockoffs are completely allowed. Game mechanics are not patented, art styles aren’t copyrighted, and there’s no legal protection for concepts in any way, shape or form.
If, for instance, Immortals: Fenix Rising were to use a character model that’s clearly a clone of Pit from Kid Icarus, which totally wouldn’t look out of place in Immortals world, then Nintendo would have something to hit with a C&D. As it is, though, there’s nothing actually copied.
Consider Queen’s Under Pressure, compared to Vanilla Ice. That baseline is so clearly copied straight from Under Pressure, but by making one minor insertion to the base line and having the rest of the song be so completely different, there is no copyright infringement.