If Hamilton has not been driving the fastest car would he be so successful

May be...

Kovalainen is in his 1st season in a McClaren. Besides he's 2nd on the grid today and a cigarette paper behind his team mate timewise.

Many had criticized Hamilton saying he has an unfair advantage of driving the fastest car than giving him the due credit for his confidence and skills. Kovalainen is driving the same car too, but why isn't he faring better than Massa or Hamiton?

yes

He won the British GP by over 1 minute. No car currently on the tracks will give you that much of a lead without you being an ace driver.

A fast car isn't worth anything if you don't know how to drive it properly.

It has always been the case in motor racing that the best drivers end up driving for the best teams.
Of course he would not have been as successful in a slower car.
That's why he wants to drive for a top team, that's why everybody wants to drive for a top team, because they're looking for success and they need a top car and team to do that.
Don't listen to people saying he only looks good because he drives a McLaren.
Those people couldn't make out an F1 talent out of a bunch of pensioneers.
It's like the performances of the drivers we see on the track are absolutely meaningless and it's only the car and nothing else.