There is no post-game content for Mario & Luigi: Partners in Time; Mario RPGs tend to be very scant in it in general. The only thing you could do when you reach a certain point in the game is to grind for beans and buy badges from the Fawful shop. Even the game itself resets you to the last save point before the sequences of the final boss segments.
That's pretty much how it works in every Mario RPG, actually. The only exception I can think of is Super Paper Mario (and maybe Mario + Rabbids but I don't know if you can count that as a Mario RPG? Sounds like a good debate though lol).
Also Paper Mario: The Thousand-Year Door. I honestly think they were onto something giving those two games explicit post-game content (ie, saving after the final boss and that save file being then set after the conflict is resolved). But sadly they just abandoned the idea after that. Refighting the final boss is nice but that's the kind of thing that should be available in a boss battle arena, like in the M&L games from BIS onward (which did not include the final boss as a direct result of there not being saving after the final boss)
it's not that weird for me. mario rpgs are starter jrpgs and are short, but very straight-to-the-point, linear, focused experiences. we're not going to get 100+ hours epic sagas with tons of sidequests typical of the genre with the mario franchise.
that being said i would love for mario rpgs to have actual post-game content regardless. most post-game content and sidequests are just tedious fetch quests (like the trouble center in ttyd or koopa koot's favors in 64). jrpgs and other adventure titles (such as majora's mask) have their own stories embedded in optional side quests for example that don't really tie into the main story and i think having little story arcs like that would be fine and dandy and fit nicely in a mario rpg, and could also expand on side characters such as mario's partners as characters.