Pikipedia:April Fool's Day/Olimar's notes in Super Smash Bros. Ultimate

In, a player playing as Captain Olimar can make him take notes of the fighters, interrupting the action on-screen with the notes being written. These notes are usually humorous or informative of the Pikmin lore, in a similar vein to Olimar's journal.

Activating
To activate the notes, the player must press the down taunt button for a single frame whilst playing on the Distant Planet stage. Much like other Smash Taunts in the Super Smash Bros. series, Olimar will crouch down in a pose for a few seconds. If he is interrupted during this state, the activation will fail, and the player will be unable to try again in the same match. After Olimar gets up, a message box, similar to the ones in Olimar's journal in the Treasure Hoard screen will appear on top of the battle, obscuring the bottom part. Olimar will speak in time with the message being filled out, using his random phonemes from the Secret File videos. Olimar will write about one of the fighters he is up against in that match, picked at random.