Should I make a custom moveset to match my amiibo's?

What I mean by this is if my Kirby amiibo had Giant hammer and Upper Cutter, should I create a custom character (near where you make mii fighters) that also has Giant hammer and Upper Cutter, so I could fight the amiibo using it?

I do that.  How better for your Amiibo to learn their moves than to face someone who has the same moves?

It's recommended, as some custom moves work in situations where the regular move won't or vice versa.

A simple example is Mario's Fireball. The regular one falls with gravity, so you can edgeguard with it. If Mario is given the fast fireball, which travels in a straight horizontal line, then Mario might try to edgeguard with his version of the fireball. It won't work, but he'll try it because it worked when you did it.

If you have the option, do it.