My IQ is less than 160...please explain this to me.

The only thing that I can figure is that very few people know the original composition, as Ravel's version is far more popular. But I could be missing a lot, as this joke doesn't seem very funny to me.

The composer’s name is Modest Mussorgsky, so it’s a play on the word “modest”.

Ah... thanks. My suspicions were confirmed and this still isn't funny for me.

His name literally is "Modest Mussorgsky"

Right. And he said something modest. ...?

just taking a Promenade through the comments

He said there was nothing great about a famous piece of his.

I didnt even know it was a classical piece, I thought it was an Emerson, Lake and Palmer thing

Right. So a modest guy said something modest.

That's not a joke. If he boasted, that might resemble a joke. But modest people are modest. That's not funny.