Dumb? Nah. Naive and completely ignorant of other points of view? Yeah.
It seems like he really assumed Corlys was on the same page as him with thinking that marrying someone that young just wasn't feasible. He definitely didn't consider that the pairing meant more to him than just a routine joining of houses, though.
What's crazy is that a lot of Viserys' problems are ones that could easily have been solved by talking to people, instead of relying on Otto and that Maester to handle the heavy lifting. If he just talked to Corlys about his choice before announcing it, he'd probably be alright. Could've easily spun it off as respecting his house and wanting to link them in, but wanting to do right by his daughter by giving her an opportunity to wed someone closer to her age or something.