Grass is not toxic for cats, and most don’t show much interest in it. However, some cats enjoy chewing on it, and it is generally accepted that this behavior is harmless. Most cats only nibble at the grass and throw up occasionally (once every 2-3 weeks), but some cats eat a substantial amount of grass regularly, and vomit every few days.
If your cat is vomiting or has diarrhea, the cause is not grass. Your cat should be examined by a veterinarian to determine the cause of these signs.