Does Scorchaw deal bonus damage to weak foes?

This is a passive, so it needs no setup and no uptime management. It reads the target rather than anything you do, which makes it a check on your target priority instead of your rotation. Combat runs on an elemental rock paper scissors system, and fire attacks already hit harder into Ice and Grass. Those are the enemies the passive turns on against, so picking Scorchaw into an Ice or Grass pack is most of the decision.

Against anything outside that matchup the passive is dead weight. It does not fire on neutral targets, and it gives you nothing extra against enemies that resist fire. Bringing it into a fight with no fire weakness in it costs you the bonus entirely, which is a reason to check the enemy composition before you commit to the team rather than after.

Stacking it with the skill kit

The recommended skill setup is Fire Kick and Burn Breaker, and both pair well with a passive that is permanently live. A successful dodge with Fire Kick raises your damage for the next 20 seconds, so the window after a dodge is when you want your big hit out. Burn Breaker is that hit. Charge it fully and it consumes every point of EP you have left, with the bonus damage scaling to how much it eats. Dump it into a fire-weak target during the dodge window and all three sources land on the same swing.

Other fire units with the same idea

Scorchaw is not the only creature built this way. Emberpup's trait, Scorching Flames, also increases its damage against enemies weak to its element, and Emberpup's dodge skill has the same shape as Fire Kick. Timing it correctly makes the follow-up attack deal 120% damage and grants a 20% damage increase for 20 seconds. If you already run Emberpup, adding Scorchaw does not give you a new trick, it gives you a second body that wants the same targets.

Not settled: How the 15% combines with the base elemental advantage against Ice and Grass is not settled. It is not stated whether the two multiply together or simply add.