It depends:
The sprites for Pokemon are readily available online, and as far as I'm aware, those sprites alone aren't illegal. Implementing the sprites in any sort of project without the express consent of GameFreak and Nintendo is very illegal though if you intend on using them in a way that earns you money. I don't think your potential employer would get you in legal trouble, but it would be a black mark against you if they were even considering hiring you.
It's best to go in with original work, as employers are more interested in seeing what you can do on your own without half the work done for you already. Design is half the battle when starting your own projects.
Really though, your portfolio should be fleshed out with only...