
I think that, first of all, the word stupid should be swapped with ignorance. For a normal human being, what makes the biggest difference between being smart and stupid (from now on - ignorant) is curiosity. Of course this is just my personal opinion, but curiosity is a driving force that leads to doing a lot of research and thereby learning.
Our brains pretty much have the same potential and processing power. Let's use the analogy of having a PC. If you give 10 equally powerful PCs to 10 different people, the difference between them will be in the way those people use them. Someone may have 100 GB of music, movies, may work with many programs, or may play many games, while some other guy may only use it for checking his e-mail. What I'm trying to say is that all people have more or less the same (cognitive) potential, it's just what they do with it.
Understandably, we all have different tendencies and are interested in different things, and a lot of that may not be a conscious choice, but the key point to remember is that wanting to learn is perhaps more important than the act of learning itself.

Think about it, if you want to learn something, be it a specific field such as singing or drawing, or just some general information like politics, that desire to learn about it will take you to the place of looking up relevant info and if required practicing what you learn. But, if you don't have the desire to improve and learn more, you'll most likely settle down for what you know already or what you learn "by accident", and the result of that is wasted potential and mediocrity.
If we consider that state of being smart, knowledgeable and just able to do stuff in general, you can only get there if you work at it. It's something that comes as a reward for your efforts by default. If you start working on your rapping skills for example, you will inevitably become better at it. And the more you work the better the result will be.
So if you let's say want to increase your knowledge in politics, the more you research material about the field, the more you'll know about it.

I guess what I'm trying to say is this - you shouldn't be afraid of becoming something great. Something that you most probably would like to become. You only need to put in the effort and the results will come by themselves. Of course, you need to stay open-minded, you need to question and criticize things, but if you have the desire to learn you're good to go. Remember that, just by wanting to improve and looking for ways to do it - you've already won.
The problem with most people, as I said, is that they don't even look for ways to improve, and that's something that is hard to "repair". And just to summarize, and make sure that I haven't contradicted myself or something, I have two main points:
One, the desire to learn is extremely important. Two, the consequence of this is that if you have that desire you will inevitably learn what you want to learn so the only thing you need to do is go for it.
Bonus hint: Don't be afraid or mad at failure. Failure is just one way of sorting things out and finding out what works and what not. At the end of the day, it only helps you to get where you want to get, and you just need to be patient and keep going.g