It really depends on what's important to you from the games. I think there's no arguing that Mask is a tonal departure (as well as a departure in so many other ways) from the other games in the series. Then again, KQ7 is also a bit of a tonal departure, though it fits overall with the first 5 better than Mask. KQ6 is probably the best mix of the light-hearted tone of the first 5 games and the more "mature" (blech) tone of a romance novel, but it strikes a good balance.
In my opinion, Mask's radical tonal departure from the earlier games was a bad decision, compounded by the 3D graphics that weren't yet technologically up to the task of handling an "epic adventure" in the way I believe Roberta and the other designers were hoping they'd be. But yes, the darker tone is a big part of why Mask doesn't feel like a King's Quest game, not to mention all the other gameplay differences.