When it comes to scoring, Garnett is held to a standard that has always puzzled me. It's like, the labels that have been put on him by some (i.e. not clutch, must be a secondary scorer, etc.) simply persist no matter what his actual output on the court.
To answer the OP, yes, KG can definitely be the leading scorer on a champion. As has been pointed out, he was really the leading scorer on the '08 team. In the regular season he scored more when he was out there he just played fewer minutes, but in the postseason when he played the same role with normal playing time he was the leader.
Another way to look at it, Garnett is pretty much exactly the same scorer as Tim Duncan, especially once he made the full time move to PF in 2002. From '97 (when Duncan was drafted) to '08 (the site I'm using hasn't updated yet), Duncan has averaged 21.6 points on 50.8% FG shooting and KG has averaged 21.7 points on 49.4% FG shooting. Since moving to full-time PF in 2002, KG averaged 22.1 ppg on 50.5% FG vs Duncan's 20.6 ppg on 50.6% FG. Essentially, on offense, KG and Duncan have almost the exact same output. Whether one operates more out of the post or not isn't especially relevant, since their individual output, their contributions to team offensive efficiency, and their individual efficiencies are all as even as 2 players can be.
Between them, Duncan and KG have been the leading scorers on 5 of the last 11 championship teams. And the Pistons also won one in there with a team that KG would have easily been the leading scorer on. His scoring output is just fine for leading a championship team.