I think Ridley has a slight advantage in this MU.
Ridley has the superior disjoints and safer KO options. Zelda, while powerful, is risky and it's difficult to land her moves, due to their weird hitboxs and properties. She struggles against characters with safe options and being light herself, can't afford to be too risky. She is overwhelming in advantage, but struggles in neutral and her disadvantage is horrible. In contrast, Ridley has a great neutral and relatively safe KO options. Being able to pressure and remain safe in neutral, is going to put this MU in his favor.
Play it safe and focus on keeping Zelda in disadvantage. Side-B can punish Phantom charges at mid-range, so stick close to her and punish those attempts. If she can't easily use Phantom, then there isn't much she can do about those disjoints of Ridley's. As such, keep walling her out and slowly push her toward the ledge and off stage. There isn't any easy way to challenge her recovery, so use Ridley's projectile to pressure and rack up damage. If she does manage to make it to the ledge, focus on trapping her and punishing her getup options. She is slow and floaty in the air, so focus on covering roll and normal getup/getup attack and don't over commit. That way if she does take to the air, you can be ready to chase and catch her landing.
Don't fish for kills too hard, that's what Zelda wants you to do. Instead, force her into the air or offstage and keep pressuring her landing. If you can force a bad landing or read a getup option, Ridley has good and quick kill moves and can easily KO Zelda at around 100 or above. If you find it difficult to do that, as long as you can keep hitting Zelda back into disadvantage, you will get it eventually. When in doubt, hit her back offstage or in the air and repeat the process, until you're comfortable committing to a KO option. The burden is much greater on Zelda getting back to the stage, than it is on Ridley edgeguarding, so there is more chances for Zelda to make a mistake.
Things to watch out for, are Zelda's Up-air and edgeguarding tools. Ridley has no way of challenging Zelda's Up-air, so either get good at juking or retreat to the ledge. If the Zelda is particularly good at Up-air, not only will you be juggled, but you are in danger of losing a stock. Up-Tilt is also something to watch for in this situation, as it is good for catching landings and can hit the bottom platform.
Offstage you want to mix up your low recoveries and never recover high. Phantom and Din's Fire are very good for catching high recoveries, along with Up-air and Ridley being so huge, makes it even easier. Go low and make sure you use Ridley's jumps and Up-B deceptively. Ridley isn't exactly easy to edgeguard, but isn't exactly hard either. If Zelda understands the timing, Up-B is easily spiked, since Zelda's Dair is intangible and Nayru's Love, while somewhat weak, is easy to hit with and also has invincibility frames and may put you in a bad position. Don't use Side-B as a recovery, unless Zelda is below the ledge. Phantom is very good at catching it and it is slow enough, Zelda can challenge it in the air.
So in other words: oppress with neutral-> force into disadvantage -> force bad landings and getups -> kill bad landings and options -> rinse and repeat.
Ridley is safe, simple, and effective, so you should play him that way. Zelda's powerful kill options, outside of Fsmash, are risky and easily punished. Ridley will have the advantage in high pressure last hit situations, so you can afford to be patient and wait for a whiff or opening. Don't give Zelda any openings to punish and remember that flow chart and you should be fine.
I personally think Zelda is the better and more versatile character, but Ridley has tools that easily exploit Zelda's weaknesses.
