In Wolverine and the X-Men #3 (I think) Iceman uses his powers to create a large group of ice monsters to help in the fight against Krakoa.
Is this the first time he used this ability or has he done it before?
During his time with X-Factor, but in the pages of Thor (issues 377-378, March and April of 1987), Iceman was captured by Loki so his powers could be used to gain control of the Frost Giants. Loki enhanced Iceman's powers so much that he lost control of them and had to wear an inhibitor belt for some time.
Later, when Iceman was back with the X-Men (issue 314, July of 1994), Emma Frost possessed Iceman for a short time and used his powers in ways he hadn't imagined before. This is the point where Iceman starts freezing his entire body solid (instead of just encasing it in a protective shell of ice), increasing his size, and giving himself a spiky appearance.
I remembered a noticeable "shift" or a blossoming of Iceman's powers start around the time of Uncanny X-Men #313-314 (July 1994?) Emma Frost takes over his body and uses his ice powers in new, more powerful ways that he never imagined. I think afterward, Iceman couldn't turn back to his human form for a while and looked more monster-like in his ice form than humanoid. Iceman / Bobby Drake turned a little more darker / more introspective after that (as opposed to his happy-go-lucky classic X-men days).
Iceman has been considered to be one of the most powerful superheros and is an Omega-level mutant.
Apocalypse considers Iceman to be one of the twelve most powerful mutants on Earth. - Superhero Universe
As mentioned by writer Mike Carey, Iceman is "an Omega level mutant...[and] has powers that can influence the ecosystem of the entire world." - Wikipedia