Joker card text
"This card is only used for draw hands and pulls.
You choose its suit and value. Ace the joker after use."
The Angler is spoiled here and card text referenced above.
As the Joker text has no value except when revealed for draw hands and pulls, discarding it to the Angler is not a legal play.
Jokers can still be discarded to other abilities that allow you to discard cards.