The angle of the ship sticking out the side of the mountain is too great... they would lose a lot of space... and would have to gut out most of the ship just to create a couple of usable flat levels. If they did that, it would have been easier to dig out the Ark and have it level on the ground next to the volcano (or in the dug out cavern).
It was shown at two different angles - pointing down into the mountain, or with the left/port side angling down. The exit they would drive out of was level to the ground, but the ship itself was always left at a rather big angle from being level.... which is now overthinking a kids cartoon way too much.
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