I think its just really bad to use in FFA matches with 4 people..... which is a shame, because thats when final smashes are used the most.
There are usually 2 flavors for Final Smashes;
"Catastrophic" final smashes: (example, Olimar, Mario, Ness) They change the stage dynamics, and present a new hazard. If a move attacks everyone, it does a lot of damage, usually either 30-40 everyone, combined with a potential to get knocked back a alot. Causes a big ruckus on stage. Adds excitement.
Cinematic final smashes: ((Captain Falcon, Megaman, Villager) Often involves a cutscene, usually the move hits a single player, its usually an OHKO or very high knockback.
Peach/daisys final smash is in a weird spot of neither category. Its sure, but theres no new 'hazard' being presented.
You have to play the game of trying to be close to an opponent (to inflict more than 20%), the sleep duration isnt that long, and you only get one attack from your base set of attacks to use. The best option for damage is like.... fully charged frying pan for an extra 26%, and even then, it might not even kill.
You wont even have time to attack all the players with the short sleep duration, either.
So in a 4 player matches, where final smashes are most often used, Peach/Daisys attacks just fall short. bleh.