Most players obviously prefer a character that you can take directly into the thick of the fighting, which happens to be the majority of them. Iron Citron isn't a "bad" variant, he's just really different.
Most Players get used to using Citrons in a frontal assault capacity so it becomes ingrained to get into the thick of the fighting so when most unlock Iron Citron they default into that playing style because it suits every other Citron pretty well and then you get slapped with the jarring realization that you're getting yourself wrecked because the single shot and slow reload time give the Zombies plenty of time to maneuver and take you out so a lot of people simply label him as a "bad" variant rather than investing any time in figuring out the best way to adapt to him.
He's definitely a character that you play in order to rack up Assists rather than Vanquishes at first, learning to work at longer ranges, figuring out the best vantage points in the map to take up post and you definitely have to work to get good enough with him to be able to trigger his Iron Mode in Multiplayer.