^She's good vs big enemies/bosses who don't get launched away. You can deal 60-80% in one jump/double jump by sweet spotting LKs. Reflect-able attacks are easy to see coming which makes Nayru's a lot better than in vs mode. She's good for continuous pressuring then a powerful followup to finish enemies, generators, Floows. Din's can't be blocked by most enemies and the loaded stage extends a good distance beyond the screen; it's damage is good for a projectile and Zelda can snipe well.
Zelda does lose her 'greatness' in very hard/intense difficulties (Ganon too). Her slow speed/mobility and light weight kill her.
Great wasn't the best word. I meant more of comparing how good a character is in Subspace as opposed to vs mode. For example, this would be my Subspace (very hard/intense) character tier list from the top of my head:
S
Link (he only fails on Halberd exterior)
A
C Falcon, Wolf, Fox, G&W, Yoshi
B
Zelda, Ike, Pika, Pit, TLink, Falco, Wario
C
Samus, Ganon, Luigi, Lucario, Bowser, DK
D
...and so on.
Character values for that kind of game are very different from competitive smash. I think that might have thrown off the competitive balance in part

.