Well I would be more concerned with any pesticides/fertilizers that are used on the lawn/grass. Most grass should be fine, though there are some kinds that are really hairy looking/feeling (not usually in a lawn though) that might irritate their eye if they rubbed on it. When the grass starts to make seeds, depending on the type, it could have some pokey bits that I wouldn't want to put my chameleons on. Most should be okay as long as they aren't treated with anything dangerous for the animal (especially if it's a veiled that could possibly take a bite.)