Yes—falling asleep during a session is perfectly fine, and the session will still be effective. In fact, the state of drifting off to sleep is very supportive of hypnosis, as it makes you even more receptive to positive suggestions.
Even when your conscious mind relaxes, your subconscious continues to listen and absorb the guidance, allowing the process to work just as intended. So feel free to get comfortable, relax, and let the session carry you into sleep.