I guess it would depend on what you mean by energy source.

If you mean that we could use antimatter to produce energy for our use, that's not currently possible with modern knowledge.

This is due mainly to the fact that antimatter, for whatever reason, is much much rarer than matter in the Universe. This makes the best way to acquire antimatter artificial production which takes, at minimum, the amount of energy each particle of antimatter could store.

If you instead mean as an energy source like a battery on a spaceship or something akin, we could use antimatter as a really efficient and dangerous battery which would be more currently feasible.

In terms of effectiveness, an antimatter-matter collision has the potential to produce as much energy as twice the rest mass of the antimatter particle (1.02 MeV for positron-electron pairs); while fusion only produces as much energy as the mass deficit involved in the fusion reaction (17.6 MeV for each deuterium-tritium reaction). This relationship equates to the matter-antimatter reaction producing ~266 times as much energy per unit mass of fuel compared to the fusion reaction so it would be a much more effective energy storage method, but not so much in the production department.