For this matchup, I will say it's 50:50. Having practiced this matchup heavily, I believe it's all dependent on momentum. If Ganon is able to close the gap, he can rack up serious damage and potentially KO; Side-B tech chase is especially dangerous. It's very dangerous to approach him offstage as well, because if you come at him horizontally that's a Ganoncide, vertically that could be Up-B, F-air, U-air, N-air, even Down-B and Down-air if you end up on him or below. Zelda's aerial game is basically gone, because of high end lag on all aerials (the lowest being 21 frames, I believe).
The only way a Zelda can approach Ganon is to not approach. In order to be successful with Zelda, heavy spacing is required. That means use jab's disjoints and knockback to punish bad approaches, string Up-tilts at EARLY percents to rack up damage (DO NOT try to string at higher percents, because this will mean a D-air with the strongest meteor in the game and transcendent priority is coming straight to your head), Down-tilt at mid percents to lightning kick, Naryu's love to punish roll approaches, Elevator Oos is also very effective. Din's fire should only be used when Ganon is offstage since it's so risky in the neutral. Another quick "combo" mix up you can use at higher percents, jab to Farore's wind. (Only works if they don't tech.)
If Ganon has you offstage be very careful about predictable recoveries, try to recover where Ganon can't; it's less likely he'll kill himself to get an unconfirmed edge guard on you, unless he's up on stocks. Utilize love jumps when you can, in order to mix up recoveries and to possibly stage an offense on him. Recovering with Farore's onstage is extremely risky because of the lovely endlag. Also remember, Ganon is only very slightly slower than Zelda, so don't think you can run circles around him all day.
Overall, it's all about stage control and spacing. You HAVE to respect the damage he can dish out, but you CANNOT be afraid to attack and punish. Make him respect your damage output by landing combo after combo, punish after punish. Never yield to his approaches, keep your distance, stand your ground. Giving him stage control gives him the match.
This is my very first post, so I'm sorry if it's a jumbled mess of words and stuff. :^)