Is Earth mostly liquid, like a water balloon?

Technically speaking, no. The mantle, which is solid, comprises about 2/3 of the Earth's mass. However at a planetary scale solids are not rigid enough to maintain their shape, so the Earth is closer to a liquid held together by gravity than to a rigid solid object. See this simulation for an interesting demonstration of its properties: https://youtu.be/kRlhlCWplqk