I won't say it's very hard for most humans with a brain. It depends mostly on the goal and the time you can invest. If the goal is to modify, or write a simple app, it could take a few months, 1 year, to learn the basics. For a complex hacking project (nintendon't, finding exploits), it's a complete different story : years of good knowledge in c,c++, assembly, and hardware.
It'd be better to start with a simple C tutorial, at least the first chapters to see how the language looks like (the variables, functions etc.).
You can also skip this step with the tutorials from
codemii.com. They explain some basics of C, your 1st hello world, and what is wii specific.
Wiibrew has a few tutorials too.