Question

Example: A pikachu only knowing electric moves.

Comments

Yes, if you want to get destroyed in battle. :)

Answer 1

Absolutely not. Why?
Go! Diglett! Yeah. It is best to have all your moves of a different type, so you can spread out your coverage. Hidden Power [Ice] takes care of that by hitting Ground types that are immune to Electric.
Spread out the types, spread out the coverage.

Comments

Yes Mew, Grass is immune to Electric. :P LOL.

Answer 2

No that is horrible, a type that resist your type can come in and wreck you.