The following contains spoilers for My Hero Academia Season 6 Episode 1, "A Quiet Beginning," by Bones, available now in English through Crunchyroll
Sometimes, when the English dub of an anime has an opportunity to change the original script, they'll take it. This can be done for all sorts of reasons like matching lip flaps or localization. Sometimes, it's to include a joke that's just too perfect to pass up.
Whoever was in charge of the My Hero Academia dub's script saw such an opportunity to change the script for the sixth season. Not only did it help to sell the dub to American audiences, but it also injected some humor that could only be possible with the given setup. In this case, the dub was changed to incorporate a Looney Tunes reference.
The reference appeared at the end of Season 6 Episode 1, "A Quiet Beginning." At the very end of the episode, there was a chance encounter between the Rabbit Hero: Mirko, and All For One's top scientist, Dr. Kyudai Garaki. In the Original Japanese release of the anime, Mirko simply asked the doctor "Are you the real one?" regarding an impostor shown earlier in the episode. The English dub embellished this line by having her say "What's up, Doc? I'm guessing you're the real one!"
This is a reference to Bugs Bunny's classic means of greeting people. The rabbit will often go up to someone, nibble a carrot, and ask the question "What's up Doc?" The only thing the Rabbit Hero was missing from her homage was the carrot.
As for whether it was right to add this reference, it's fine. It may not be a word-for-word translation of the original dialogue, but it still gets across the intended message. The added dialogue is also perfectly in character for Mirko given her spunky attitude. It's an unnecessary change, but not an excessive one.
Besides accuracy, the opportunity to include this joke was simply too good to pass up. There aren't too many opportunities for a talking rabbit to address a doctor, not even in anime. The opportunity becomes even less common for English script writers who have to make sure the dialogue fits with the Japanese lip flaps. Whoever was in charge of dubbing the scene between Mirko and Dr. Garaki might have seen it as a once-in-a-lifetime chance.
Mirko's Looney Tunes reference was a rare treat for those watching the English dub. It's the kind of thing that can't happen unless a lot of stars align. For now, MHA fans should enjoy this moment in the dub for what it is, a fun little homage.