Well Cullen role in DA2 wasn't so great .Not sure it's 100% the character's fault because the plot sometimes pushed him to be a bit stupid.
He has a hard time figuring out Hawke or his /her friends are apostates.
Meredith , for me , also suffered from the whole not doing anything.
I couldn't take her seriously at the end of the game , because I couldn't really see how her work was improving anything.
I mean the templars get turned into demons , Cullen is working on it , but in some case a mage Hawke solves the problem just to be told afterward "Mages are not to be trusted"....
Then we kill the Templar with the whole "Tranquil solution" who's about to turn a poor girl tranquil and imply he may turn her into some kind of sex toy.
If you talk with Cullen Afterward , he vaguely says well maybe the tranquil solution has some merits.
There's also the fact there's templars and mages banding together to rebel against Meredith before she goes full crazy.So some templars were aware Meredith wasn't doing her duty way before Cullen realise it.
I think the main good thing Cullen is able to pull out is telling Hawke , Meredith wants to fire Aveline.
On the bright side , everyone looks incompetent! Cullen , Meredith , The Grand Cleric , The Seekers ,Leliana , The Divine!
But for Inquisition they will notice the big hole in the sky and demons pouring out and it won't take them ten years to figure out there's a problem. 