What does "Dad Joke" mean in the context of the Snowden tweet?

It is a recent expression that refers to lame, poor jokes no one would find funny:

Dad joke:

• (slang) A lame, embarassing or unfunny joke told by someone's father.

(Yourdictionary.com)

Dad Jokes:

• is a pejorative label typically used to describe a variety of failed attempts at humor involving corny puns, anti-jokes or predictable punchlines, some of the prime examples being Trolldad, Bad Joke Eel and Lame Pun Coon.

Origin

• While the exact origin of the term “dad joke” is unknown, the earliest known use of the term online was posted in a thread by B3ta Forums member rob on December 10th, 2003, which asked other members to submit “lame jokes” made by their fathers. In the first 48 hours, the thread received 15 pages worth of replies.

(knowyourmeme.com)