Is Mercantilism an effective strategy in order to make a country powerful?

It's OK early game if you have enough cities, since a free specialist in each city can be quite a bonus (especially if you have Representation). However, after the discover of Astronomy, foreign trade routes become essential (provided you have enough coastal cities). Free Market or State Property will be much more effective at this point.

...unless you're not talking about the video game Civilization 4, in which case, man, I don't know.