Okay I'm genuinely curious how in the world a BLM can have such low dps in expert dungeons ! Even if they weren't AOEing it should be higher... and BLM is so easy, especially when AOEing... oh my gosh x.x This is why I support giving advice in-game even if it isn't "serious content" because that person is likely going to try and go into said "serious content" and I guarantee they won't have a pleasant experience. Nor will the others that are forced into dealing with.... that.
BLM has a lot of options when it attacks, many of which cause people to stray into suboptimal rotations. There are a lot of ways for BLM to screw up, and it shows a lot in the DF. "Ice mages" are just one of the ways this can happen; they're just the most egregious form of it.

I wouldn't call it a simple job; it's simple to *you* maybe because you play it. Most other jobs outright highlight the next GCD you should take by making its icon get a glowy outline. BLM doesn't really do that, and the few times it does get a glowy outline (Thundercloud, Firestarter, Foul/Xeno), it still doesn't mean it's an action that should be used right away. In fact, usually it isn't.