The image seemed to be uncompressed, or at least, the relevant parts of it were uncompressed. I'd guess they had to be much pickier how what and where to compress, given the constraints of the system.
Accessing the cart was as fast as accessing RAM, and you didn't have much RAM to go around, so I imagine there just wasn't much advantage to it.
No, compression was used extensively. Just often hand-rolled and for specific things. https://multimedia.cx/eggs/nes-compression/ has a link to an RLE implementation used in some Zelda game.