Good point. Why use forced perspective for a miniature? That's belt and suspenders. Forced perspective is something you use to reduce the size of the thing you're building so you don't have to use as much space and materials. If you want to do that with a miniature, you just scale the whole miniature down.
Granted, the observation corridor does slope down a little. But I don't think that's forced perspective -- just perspective. We can see from the exterior of the ship that the hull does actually taper in toward the rear, and the upper hull slopes down. So the hangar had to be designed to fit into a tapering space, one where the roof sloped relative to the deck. If the observation corridors had run perfectly parallel to the deck, that probably would've looked wrong to the viewer's eye because it wouldn't have fit into the lines of perspective suggested by the shape of the space. After all, the audience would only ever see the miniature from the front, looking in the direction of the vanishing point. So it was probably an aesthetic choice.