This is so true. I had a college mate who was truly useless with academics - struggling with pretty much all the courses and math gave him nightmares. (But he did have the determination to retry until he passed).
Yet he could disassemble your computer, your motor-bike, your car, diagnose and fix any problem - all within a couple of hours. It was really fun to watch him work. This was sadly before the youtube era. Today, I would have taken videos of him doing this. It was amazing how he knew precisely where every part was and how smooth he was de-assembling and assembling. A mechanical savant. He always used to nail the practicals - welding, foundry, etc.
Don't assume someone is not smart because they have trouble with something you find easy. People have different strengths and weaknesses and the same person you are tempted to look down upon as "dumb" may blow you out of the water on some other task.