Why is this painting with Link from Ocarina of Time funny or ironic?
I don't consider it "funny". More interesting or "cool".

According to Dave Pollot's website, the title of the painting is Leveled Up. It features Link, from The Legend of Zelda, riding a horse with Navi over a reproduction of Napoleon Crossing the Alps.

Notably, Napolean refused to sit for the original set of paintings:


  Sit? For what good? Do you think that the great men of Antiquity for whom we have images sat? ... It isn't the exactness of the features, a wart on the nose which gives the resemblance. It is the character that dictates what must be painted... Nobody knows if the portraits of the great men resemble them, it is enough that their genius lives there.


The Link version effectively captures the spirit of the originals, since Napolean's posture of victory is one modern viewers would associate with having just leveled up. If Jacques-Louis David had been a truly daring man, he might have painted something similar while attempting to convince Napolean to sit for him.