Can I use an NES controller as a Makey Makey? I don't want to buy one to find out I can't.

The purposes and capabilities of both devices are very different, so I'd say you'd not be able to.
I don't think you can connect a NES controller to a Makey Makey either.

Good question, that made me think.
Short answer: No.
Long answer: Theoretically, but it would be difficult. First, assuming you meant an OG NES, you would need an adapter to USB, but they seem relatively hard to find. But that's not all! You would need a Gamepad Adapter that can remap controllers, such as the one by 8BitDo. There are digital ones, but I've had bad luck with those. THEN you would need to find a Makey Makey project that predates the extension, and then it would work.