Some players play dubious moves in the opening even as early as move 1 just to throw their opponent off. That being said, your opponent clearly doesn't know what they're doing and got themselves fools mated.
WTF is the Barnes Opening?

Indeed, probably for the banter. Or they just don't know what they're doing. Or they do know what they're doing, and they're playing it for the specific purpose of entering the line 1. f3 e5 2. nh3 d5 3. nf2 nf6 4. e3 nc6 5. Be2 Bc5 6. 0-0 0-0 (Courtesy of Wikipedia) which is still bantz
Just checkmated someone in four moves against the Barnes Opening. Why on earth would someone play that? For the bantz? https://www.chess.com/game/live/31591807815