*starts chicken dancing*

How do you trigger this event after the pecha berry gift

During the post-game you mean? I don't think you can.

Wait... this is only DURING THE MAIN GAME

WHYYYYYYYYYY

interact with the decoration at Peachy's, and it should trigger the Mochi Mayhem epilogue, i think. The item automatically disappears from your bag once you start the epilogue

you also need to have completed both The Teal Mask and The Indigo Disk