No. The poké ball is typically sufficient enough to keep a pokémon in line. Often, it’s seen as more humane because while a leash restricts a pokémon’s range of movements, a poké ball allows them the freedom to go about as they please while still enabling their owners to pull them out of harm’s way or keep them from doing anything damaging. Of course, most pokémon paired with a poké ball are acclimated enough to human contact to behave well in public, and most trainers who own pokémon that are aggressive or otherwise not well-adjusted to human presence often know better than to force their pokémon to accompany them through high-traffic areas.
That having been said, all pokémon are legally able to walk around with their trainers in public; it’s just that there are some pokémon that are not socially acceptable to keep by one’s side. (Muk, for example.)
The Pokédex According to Bill 