Mr. "Heart of the Cards" over here roasting a guy for not playing with a tournament competitive deck.
This is such a bold statement when almost every character has a deck revolving around a specific theme. That’s literally 90% of the people you duel sir.
So there's a difference between "fun" decks and "themed" decks in TCGs like Yugioh. Even tho Yami uses the word Theme in his definition of Fun Decks. Lots of characters in Yugioh played specific themed decks, Yugi Dark Magicians, Kaiba Dragons/BEWD, Joey dice cards. But they were tournament competitive like OP mentioned. Fun decks have no strategy or balancing involved with what cards go in.
But context is important with this one. In this episode Yami's opponent is just some guy who challenged him to a duel at the local arcade. The guy suggested a bet for who wins this duel and so will lose the bet if he loses the Duel. He then walked in with a "fun" deck against the world champion. . .
Yeah this is meant to be comedic irony and if Yami was any other character he'd be laughing at this guy's face after putting together it's a fun deck. He's not bashing Fun decks and would agree you should put cards in your deck you enjoy. Just don't go betting something on a duel against the TCG world champion with that fun deck man.