I used to think that the great paintings were all painted a long time ago, and that modern artists devote themselves to comic books, advertising, and the like.
I was in London at the National Portrait Gallery. They have a competition each year for portraits, and I saw the gallery. Some of them were pretty amazing, almost photo realistic. They make the Mona Lisa look like it was drawn by school children, at least in terms of technical quality.
As for the Mona Lisa's fame, it's just hype feeding more hype. It's good, but how do you judge art? It's clearly better made than a lot of portraits, but is it the best? There are no definite answers to those questions, but what is definite is that a lot of rich people and famous galleries would love to have that painting, and the hype just feeds itself.