70hp in a four door sedan would be horrible. In a smart, it's good enough. Remember, it's only half a car, so it's like getting 120hp or more in a four seater. HP isn't as important as power to weight ratio. The smart is 1850lbs more or less (2350 with two passengers). I read somewhere that it has the same power to weight ratio as a Toyota Camry 4 cylinder.
Of course, if you're embarrassed to tell people you only have 70hp, find another car and forget about saving gas... in a gas-only engine, HP is inversely proportional to MPG.