In theory, if you only use fireballs to punish ending lag, that would work just fine, but that's not the best way to be using fireballs. Lucas is supposed to counter the best way Mario can use fireballs as active space control tools, since trades with fireball absorbing and Mario doing an aerial are not usually favorable (he heals more damage than he takes in this situation normally). Using fireballs "intelligently" might not even work if he has the balls to just take a hit and still get a net gain of health.
And he WILL be looking to absorb fireballs since he can magnet cancel, which you can obviously bait, but still, he WILL be absorbing fireballs if you implement them, and chances are you need to implement them in this matchup.
Most of us limit ourselves too much from using fireballs because of this fact alone, so if you can make that a non-issue and STILL get your "free" followups, that will make the matchup a lot easier. Most of the damage Mario deals is through his aerials and Smashes, so getting less damage from random hits with fireballs isn't much to cry about, especially if the registered stale fireballs is increasing your overall damage output from your other attacks.