I don't think you really need advice. But here is my 2 cents.
I personally think you already know how to play zelda from watching the vids. And if your asking for help I guess your losing? Well if your fighting high tiers it's gonna happen >.>; If your atleast keeping up with them that should be satisfactory enough. That was a pretty solid fox you played against, and you had good accuracy.
All that's left on my opinion for you is to just practice, and explore ways to counter what pwned you.
Vs say Marth, or Ganon, Where they have dangerous range with their F-airs and more vs your zelda. You can't be spamming kicks. Their moves have to be baited, So fast ground moving, and lots of running away with fake approaches is good. So is doing a real approach sometimes. And Grab does have it's uses, Mainly because if you nearly 100% never use it. They'll shield all the time. So when you finall get into their defenses Grab is sometimes good, IF they are scared of a possible kick or dash attack.
Dash attack is nearly as good as the kick, when you approach with it at the right time between aerial or ground attack intervals. Mixed with the grab so they won't shield 100% of the time. And the reason I'm saying Dash attack is nearly as good. Is because it does lead into kick. Now of course it isn't gonna lead into kick vs marth at like 60% >.> pple like marth it only works 0-18% ish. When they get too high it's just up-air, not much left you can do. You already know all of this of course I'm sure.
But there really is a such thing as too many kicks. Especially if your forced to approach with it. Kick has range, but you really gotta be doing it in place while they are in place, or retreating with ur b-air, while they are approaching. Otherwise your gonna get ganked. Zelda isn't the only person who can do cool stuff out of shield.
I personally don't recommend jabbing Marth unlesss your expecting a grab. Hair thin spacing with kicks/dash attack/Grab if he's approaching with aerials. This is so dangerous because it's Marth and his power as at the tip, but we Zelda's don't really have a choice :\
If he's ground gaming with dash attack, smash attacks, Grabs, and Forward-B's. Well there's a problem here kinda. You can't play your shield game too much, leads into grabs. Can't run away too too much, his dash attack can catch you. But if he trys to dash attack you as you run away, shield. Free b-air if he's set up right. Dash attack has long range, but long lag, bah you know this. If he's too close on you with moves like Forward-B on your shield, or dumb enough to approach with Down-air when you have full shields. Naryu's love out of shield, however in most cases you can b-air/f-air, or UpSmash out of shield if he's really really close.
Really though if I was Marth fighting Zelda and I wanted the win. I would camp.
I would stay in one spot. And As Zelda approached i'd spam retreating shffl'd aerials with ground moves. If I felt unsafe from a kick, i'd shield then jump out of shield with a F-air. IF it was a l-canceled Kick, I'd Shield grab after the ground move. If she l-cancelled into grab... well most likely i'm boned a few times. Yes Marth can Sheild grab her kick, and grab her before she grabs him, but this has to be anticipated, There is no way you can see her Start grabbing, then choose to grab and beat her. If she already started, and you are just now hitting A to grab as Marth, it's too late. Or you know full well your opponent doesn't l-cancel into ground moves. But once I realize it's being used alot I'd start back ground dodging after shield. Which Zelda can mix up with naryu's love. But If your Zelda and you get F-air'd and popped up, your about to take a few more aerials.
But I would play Marth like I "Should" play agaisnt peach. Camp, Camp, and CAmp. And Sadly Zelda doesn't have a good projectile to make Marth stop camping. Unless it's a huge stage like Jungle Japes. Where you can be somewhere far and safe enough to Din's fire him w/o him getting to you first. Jungle japes marth has to be jumpe and such to reach you. He can't just dash at full speed.
But since the marth you play does not play that way.
By all means bait and bait. And b-air after his misses.
EDIT: In a sense Zelda doesn't have to come to you and she can camp as well Assuming ur the Marth. But it's really about who has the better patience and who has higher% or less stock. Most likely Zelda, and your gonna most likely be forced to approach sooner or later. All Marth needs is a small lead on you then he's good for camping.
P.S. I know it looks like Marth advice, but know your opponents options vs you, and you can think about how to avoid them.
EDIT:#2
Stages to possible avoid Marth's gayness.
Rainbow Cruise. He'll be forced to come to you, however As the Stage goes up. It's a sad time because Marth's up-air could potentially pwn you. But when you hit the top area of the stage you can stay in one spot and make him come to you. (If he's the campy type)
Vs a Aggressive Marth small death zones.
PS Part 2. PSst Light shield is good vs Marth sometimes. Use it ^^d
Also, Zelda just doesn't die on those stages, or at least not as fast.