Why is Jupiter visibly squashed?

Because it turns once in under ten hours. A ball of gas that size spinning that fast throws its equator outward, and Jupiter is about seven per cent wider across the equator than pole to pole - a difference of some 9,000 kilometres, which is visible as a distinctly oval disc in a small telescope.

Every rotating body does this to some degree. Earth bulges too, by about 43 kilometres, which is why Chimborazo in Ecuador is further from the centre of the planet than Everest despite being much lower above sea level. The effect is far greater on Jupiter because it is gas rather than rock and has nothing holding its shape but gravity.

The rotation also drives the stripes. Fast spin stretches the atmosphere into bands parallel to the equator with neighbouring bands moving in opposite directions, and jet streams tear along the boundaries. The Great Red Spot sits wedged between two opposing flows, which is much of why a storm can last centuries rather than days - and Jupiter's day is not even a single number, since the equator rotates several minutes faster than higher latitudes and astronomers use the magnetic field's period as the official one.