Do grades define intelligence?

No not necessarily.

You will need to be able to do certain tasks (solve math problems, create art, etc ...), but even if you know how everything in the book and more and ace tests, but don't do in-class stuff (HW, labs, daily tasks, etc.), that can drop your grades significantly. Thus even if your grades are average that doesn't mean that your intelligence is average.

No because some people actually cheat when taking a test so it's not their intelligence