Perhaps... but Cullen hasn't done something that was genuinely "right" in such a long time that I couldn't really consider him to be a hero in any way, shape or form. He certainly has the potential to be a good person, but he hasn't yet realized it.
See this is what confuses me, people wonder why he didn't do something sooner...
He was acting under direct orders from his superior. Being second in command does give him the right to question his superiors motives and remove them of command IF they had hard proof that they are no longer fit to command.
Cullen may have had his doubts about Meredith's motives but seeing the state of the mages in Kirkwall its a safe bet he probably felt she was just being extra vigilant and probably justified.
It wasn't until she completely lost it, calling for the death of Hawke when he/she wasn't even part of the circle and, they had already had plans to just arrest him/her. That's when he could clearly see that this wasn't about the safety of Kirkwall and the calming of a catastophe, instead it was just Meredith losing her marbles. and that's when he had all the proof he really needed. So I think he acted at pretty much the earliest possible time.