I think it's interesting that we see Icarax fail badly in his fight with Teridax, tiring himself out by physically assaulting his heavily armoured form and allowing him to take a swift victory with his powers.
And then later in that same year, we learn that Icarax went up against another incredibly powerful character in the form of Botar, and rather than throw himself into a fight, everything seems to indicate that he just immediately goes for his magnetism powers and crushes him with a wave of his spidery fingers.
Icarax gets viewed as a dumb brute because that's how Mutran sees him, it's how Teridax sees him, and pretty much every other Makuta agrees with him, but the guy isn't stupid. He's clearly learned from his mistakes fighting Teridax (true, he underestimates Ignika later on, but the guy's probably killed hundreds of Toa before, he bodied all six of the Toa Nuva and took Gali's Nova Blast to the face and all it did was relocate him, I can forgive overconfidence when facing a Toa), he recognises that going to Karda Nui is a bad idea and initially refuses, he figures out that awakening Mata Nui is a bad idea, even if he's not aware of the full details, and he's the one that convinces Krika to actually rebel against the Plan rather than just giving vague warnings and being sad. Sure, he's not doing it for good reasons, and he's basically just trying to get someone else to endanger themselves for his benefit, but my point isn't that Icarax is nice, it's that Icarax is smart.
If Icarax has a weakness, it's being focused on achieving victory solely through martial conquest, and to his credit, he's very good at that. Before he gets devolved, he's probably the most skilled warrior in the entire MU, and even after he's injured, it takes three Makuta working in conjunction to actually defeat him, as he would've killed Gorast and Vamprah if not for Mutran's interference.
But that doesn't mean he's not smart. He might not be a super-genius like Teridax, but he was clearly sharp enough to spot what the rest of the Brotherhood missed, and he had the drive to do something about it.