Like...live grass? Yes, live grass would be okay. If you're talking about fake grass or reptile carpet, there are other substrates that are more suitable.
Depends where the grass comes from. If you are getting it out of your yard. I would avoid it due to the fact that there could be any number of things living in it. If you are going it controlled (indoors from seed) then I would think it would be ok for certain reptiles.