When viewing a mushroom on a Moo-Shroom the mushroom when viewed at certain angles relative to the Moo-Shroom's rotation will become distorted.

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To replicate it just fly directly overhead one of the mushrooms, it will be elongated if done right.
If you wish to view this effect on the ground, move the player so it is looking directly at the side of one of the faces of the mushroom, then one of the mushroom sides should be vertically elongated.