It's humorous because it replaces the word "vida" (life) with "stool," which is a piece of furniture. This substitution creates a nonsensical yet funny statement.
So, "Viva la stool" is meant to be a humorous and ironic way of saying something like:
* "Long live the stool!" (As if stools are something to celebrate)
* "Life is good, especially when you have a comfy stool!"
* "Let's celebrate the humble stool!"
It's a lighthearted and playful way to express a silly thought or feeling.