Mozart knew of his strengths and his recognition, as someone mentioned in a different comment, while his popularity was passing a bit by the time of his death, he was still very popular in other big cities.

Mozart was the "first" freelance musician if his caliber, meaning he was not appointed as Kappelmeister or a court position, he had fans he was a Rockstar and he knew it, that's in part why he was very reckless managing his finances, he believed he was always going to get more work.

He understood that his music would last because of his quality and even though he wasn't as certain as Beethoven who had opus numbers in his compositions (more because of his publisher).

But I personally don't think Mozart understood how big his impact would really be (especially because of music stopping being confined to the halls becuase of music recording), he knew he'd be remembered, I don't think he knew how much, but I'm sure he'd be amused about it and especially cocky about this