To be constitutional, even a limited content-neutral no-picketing zone must be defined with sufficient precision. A Kansas funeral-picketing law, for instance, was struck down in 1995 because it banned picketing "before" and "after" funerals without defining those terms. (It has since been reenacted with more precise terms and struck down again.)
I'm not sure what legislatures should do about funeral picketing. I strongly sympathize with the desire to shield the grieving, especially given how cruel and contemptible many of the funeral picketers have been; I also think little would be lost to public debate if funeral picketing is banned. On the other hand, I do worry about the slippery-slope risks from any new exception to free-speech principles. In any case, though, I've tried to explain what First Amendment law is now, whether or not that's the way it should be.