I wouldn't say dumber, but different. Well, I'm MTF and hate everything that is or was male about me, so I actually would say dumber.
But from an objective point of view, I wouldn't.
Sex hormones do change the way you think in various ways. They don't affect everyone the same way or to the same extent, and many people make many of those lie very much, but still, in average, that's true. Dumb, common examples would be (and males and females refer to the hormone levels only here):
-Males get frustrated; females get sad.
-Males analyse; females get a feeling.
-Males are quick to react and slow to think; females are the contrary. (Think, hunters will more often have to "act now, think later" than home-bound mothers)
-Males are more prone to ADHD - this could be related to testosterone or brain sex, no one knows. Or even genetics. Who knows.
-Males are more prone to be aroused without the situation calling for it than females.
Etc.
Again, those are vastly generalisations and, while true, don't apply universally, of course.
Those, I think, are a big mix of three factors, and the prevalence of those varies a lot (plus education, but since it's not biological and definitely won't change, not really relevant I guess):
-Personality
-Sex hormones
-Sex of brain
Some trans people barely feel any change short of physical ones with HRT, while others almost feel like a new person, for example.
Now that that's explained, you haven't become dumber. Just, you don't think exactly the same.
I'm not sure, but I think those stereotypes have now been vastly attributed to which toys parents gave children. But for the sake of the example because it's known and easy, let's say it's because of the hormones, like other elements could be.
Girls are good at language; boys are good at maths.
If that were true, you might have lost language ability, but gained maths ability. Not that simple, but you get the idea.
A more likely theory is that your approach to problems and learning has changed without you noticing it. You know how some people are more visual (learn with examples and charts and colours), auditory (learn with people explaining to them and every word of the theoretical rule) or kinesthesic (learn with experience, feeling, touching, trying).
Everyone is a mix of those in various percentages. And the averages aren't the same in boys as they are in girls. Could it be because of sex hormones? Why not? If testosterone increases aggressivity, it's logical to think that it might push towards more practical (kinesthesic) approaches, for example.
So if your dominant learning approaches changed, but you're still used to your old ones, and still trying to learn through those, well, you might indeed feel dumber, because you're using strategies that are no longer the most appropriate for you. But globally you're the same. You lost "stat points" in a category but put them elsewhere.
And even if, say, academically, this made you weaker, I'm quite sure you'll have gained other advantages. Increased social skills? Better introspection? Better critical sense? Etc.
In my mind, it's similar to physical changes. In a MTF's case, you lose a lot of strength and physical potential. But in exchange, you gain such a healthy skin. Give and take everywhere. Of course, if we didn't prefer the other side, we wouldn't transition, but objectively, men and women are equal, aren't they?
I hope this wasn't too much of an useless, eternal novel.
