Think of it as sitting in the first row of a movie theater. I don't know about you, but I think that's much too close.
No, it's not that in this case.
In the first row of a movie theater, you have "too much" image.
It's all there, but it's overpowering.
In the case of being too near the castle in MK (like
11290 said) your view of the fireworks will be BLOCKED by the castle.
Many guests assume that all of the fireworks are shot from the roof of the castle, and think that they are
exploding directly
over the castle. While that is somewhat true of the "foreground" pyro (it IS fired from the roofs of some of the Fantasyland buildings,) it is not at all true for the
major large pyro of the show.
The BIG explosions are fired from a spot several yards outside the rear border of MK...
well out past where the WDW Railroad rides along its northernmost tracks before rolling into the Fantasyland station.
Those fireworks can be severely blocked by the castle, the trees and other buildings at the northern part of the hub area. Stand to far to the north of the hub (too near the castle,) and you can miss a lot of the show.