Al Gore won the popular vote nationally. He won the popular vote in FL. If the election would have been run more fairly in Florida, he would have beaten Bush and been POTUS. So no, he was not a weak candidate at all.
In a very close election as in 2000, third party candidates matter. If Nader wouldn’t have been on the ballot, Gore would have won because the vote wouldn’t have been close enough to be contested. Further, Nader’s (and many of his followers) belief that there was no difference between Gore and Bush is, in hindsight, obviously plainly wrong and led directly to the biggest foreign policy blunder since the Viet Nam war…the invasion and occupation of Iraq. Those who insist that Nader didn’t effect the outcome of the election (for what I believe to be self-serving reasons) are flatly wrong and are, in part, responsible for the debacle that was the Bush W administration.
The Bush admin continued to deny that AGW was happening at a time when action would have been very helpful. Gore was the strongest candidate w.r.t. climate ever.