The Poison Spell is an anti-troop spell that damages and slows enemy troops. It is designed to help combat pesky troops that come storming out of the Clan Castle. It does only a fraction of the damage it does to troops on Heroes.
The Heroes of Clash of Clans constitute their own unique class and, as such, they do not belong to the "troop" category. They are similar in use and appearance, but they're still a special force with unique qualities. That's why the Poison Spell is not very useful against Heroes, and it will only do a fraction of the damage compared to when using it on regular troops.