While Imp is very weak, due to him being a basic 1/1 card, he should not be underestimated due to him taking benefit from Toxic Waste Imp's ability as well as activating Imp Commander's ability. However, even with Deadly, Imp still has low health, so you should either play health boosting tricks like Camel Crossing or simply let Imp be destroyed.
Usually Mini-Ninja is a better option, as Mini-Ninja costs the same amount of brains and has the same stats, tribes and class, but with the added benefit of being able to wear down your opponent's health quicker with his Anti-Hero trait. As Template:PvZH Link, you can boost him with tricks such as Vitamin Z. As Template:PvZH Link, you can give him Deadly, then use Lurch for Lunch in order to make him do a bonus attack, finishing off any plant aside from Armored or shielded plants.
Against
Any plant that can do damage should be sufficient in dealing with Imp. When Imp has Deadly via Toxic Waste Imp or even Super Stench, however, you should be careful not to let expose your key plants to him. In that case, plants with the Armored trait, or tricks such as Root Wall and Template:PvZH Link can shut him down.
His description is a direct reference to his Suburban Almanac entry in Plants vs. Zombies.
With update 1.2.11, Imp became Sneaky's basic zombie card with 1/1, due to Zombie becoming a token card. His base stats of 2/2 is now inherited by Hot Dog Imp, which costs 2 instead of 1, but has the Strikethrough trait.