Of course there are taller mountains than the Everest. Like if you consider the whole solar system, the tallest mountain is in Mars. It is about 2.5 times the height of Mt. Everest, and had it been on Earth, going to its peak would have required you to wear a space suit. It is about 21 km tall!
But here on earth, you’d think Mt. Everest is the tallest mountain. No, it is not. In fact, the peak of Mt. Everest is of course the Highest peak. So, it is the “Highest” mountain, not the “tallest”. The tallest one would be the Mauna Kea in Hawaii. Subtle differences, you see…