According to urban dictionary,
It's a vulgar acronym: WDFD = Whoop de fucking doo.
Whoop de doo is a sarcastic phrase meaning wow! great! etc.
Mary: I had toast and eggs for breakfast this morning.
John: Wow Mary! Toast and eggs!? WDFD! (Sarcasm)
I know sarcasm doesn't translate well in Japanese, but John is making fun/teasing Mary.