Readings & Meanings:
I fell down in a trap.
"Oyaji gag" [old-man pun] is a kind of joke that can't be helped.
I am dressed up nice for my date.
Kusakammuri (艸) up top, otherwise known as the Grass Crown.
洛 below. This has San-zui on the left, the common form of 水. On the right is 各, showing a leg hitting a box and meaning "each" or "every."
洛 is also a phonetic element, importing the らく pronunciation.
So think of each blade of grass having dewdrops, which must fall.
So why is 洒落 a joke and お洒落 stylish?
洒落 can also mean cool. Cool people can be stylish, or they can be funny. (Rare is the person who is funny and good-looking, perhaps?)
But I want to talk about why 落ちる (fall) and 洒落 (joke) share a kanji. This comes straight from the idea of falling into a trap. 落ちる and 落とす can mean trick. It's a short logical leap from "trick" to "joke."
Also, did you ever notice what a Japanese person does when you tell him or her a bad joke?
(Maybe I tell a lot of bad jokes, and that's why I know.)
They pretend like they're falling.
I can't find any evidence that 落 is the connection, though. Rather, it's 転ける (commonly written as こける). This also means "fall," but more like in a "overturn and fall" way. 落ちる is falling straight down, due to gravity.
You can also see this in many manga as the sound "コケ".