viserys was a coward, not stupid. he was well read and had really smart people counseling him. he knew what he was doing the entire time. he married alicent because he wanted her. he knew marrying leana was a better choice, he even verbally acknowledges that when talking to rhaenyra. he knew what he was doing to his own daughter when he neglected her, killed her mother, spent time with alicent knowing full well what otto was up to (another thing he verbally acknowledges when “firing” otto) and even tells her not to tell rhaenyra about their meetings and then marries alicent (his daughter’s best friend, therefore leaving rhaenyra completely isolated). he made rhaenyra his heir but barely talked about it for the rest of his life, never reinforcing his decision even after he had a son. he also never actually taught her or allowed her to do anything/make any decisions and when she tried to be active in the council, he shot her down. he never prepared her to rule, everything she learned was from his poor decision making. this idea that he was dumb or innocent is just another way of excusing him of the blame when, in fact, the entire dance is his fault.
