It's not bad per say but it's not particularly good either. Earthquake comes with the advantage of hitting electric types not named Thundurus or Zapdos for supereffective damage, and allows you to hit ferrothorn well without being damaged by iron barbs. It's also the strongest option against common Azumarill and Keldeo, who are currently #3 and #21 in usage and gives gyarados a run for it's money. Iron Head only seems to be useful for Clefable, Sylveon and Gardevoir, both of which have low defense stats and will likely fall to a +1 Waterfall anyhow.
- +1 252+ Atk Mold Breaker Mega Gyarados Waterfall vs. 252 HP / 252+ Def Clefable: 216-255 (54.8 - 64.7%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
- +1 252+ Atk Mold Breaker Mega Gyarados Waterfall vs. 0 HP / 24 Def Mega Gardevoir: 336-396 (121.2 - 142.9%) -- guaranteed OHKO
- +1 252+ Atk Mold Breaker Mega Gyarados Waterfall vs. 244 HP / 0 Def (Choice Specs) Sylveon: 348-411 (88.7 - 104.8%) -- guaranteed OHKO after Stealth Rock
- +1 252+ Atk Mold Breaker Mega Gyarados Waterfall vs. 252 HP / 220+ Def (Cleric) Sylveon: 238-282 (60.4 - 71.5%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
Cleric sylveon and defensive Clefable both fail to OHKO Gyarados in return, and at +1, gyarados will certainly outspeed them. The rock types iron head would hit are already covered by Waterfall, and the only prevalent ice type, Mamoswine, is weak to waterfall. I suppose Iron Head could be run to nail Kyurem-B and get a neutral option for Breloom and Dragonite, but the former is rare and if you really want to hit the last two Ice Fang is far superior.
I suppose if you want to OHKO all fairies not named Azumarill without any set up, you could run Iron Head, but Earthquake and in some cases Ice Fang tend to be better options. It's probably better to pair Gyarados with a steel type attacker. Steel types will appreciate Gyarados's ability to remove threats like Heatran, Rotom-W and Landorus-T.