Alright Mookie, just a disclaimer, I'm not an amazing player, and I've only recently started maining Marth. But I've been focusing on the Marth ditto myself lately, so maybe I can help.
vs Zutnop:
4:54: That fsmash was cheeky haha
5:10: While this was silly looking, basically what you did was completely waste your invincibility and then run in and give him frame advantage. During your invincibility is the only time you can really straight up approach, and you should do your best to get momentum back in your favor during it after every stock. As for the spot dodges, I'm a firm believer that if you don't know exactly what to do with a spot dodge, don't do it. In other words, don't rely on it as an escape or be like "crap I'm in a bad situation". You can use it to punish Marth's grab, but you really gotta be expecting it.
Your whole 3rd stock, he was spot dodging like crazy. You went for a grab in the same way a few times, by just walking up to him and grabbing as soon as you got in range. He started predicting that on this stock and really punished you for it.
6:28: Unfortunate gimp, but that's what happens when you get grabbed by the edge haha.
Overall, I think your biggest problems in this game were being impatient, and not really dash dancing properly. For the former, you just need to kind of slow things down a bit. It's hard in Marth dittos since they're so brutal, but it's very much a bait and punish type of game. If you find yourself getting punished for something, in your case running up and grabbing, you need to stop doing it fast and find another option. As for the dash dancing, a lot of your dash dances were super quick and short, and didn't really get you anything. Dash dancing in Marth dittos is as much about controlling space as it is baiting out an attack. It's a little abstract but try to push your opponent out to the corner of the stage with your dash dance.
As an aside, mix in more dtilts/ftilts into your game. Dtilt is just great, everyone uses it and talks about it, no need to cover that. Ftilt is like all the fun of fsmash without the commitment, just make sure you're tipping it and making use of the IASA frames.
7:00: Be careful when you jump, you have to watch for what he's gonna do. You could have slapped him with an fair, or if he was going for dash attack, double jump and waveland on the platform. It looks like you were planning to empty SH > grab, which is definitely a good mixup, but you did it right after a SH f-air which made it a little more obvious.
7:05: Even though you missed the grab, you did a good job avoiding the dash attack. This guy panic spot dodges a LOT on his misses, once you get better at handling that, you'll get yourself a lot of free grabs.
7:11: Coming straight down on a marth is rough. You at least want to land behind him, and you have to be really prepared with counter/fair/dair to handle whatever he's throwing at you. His spacing was good though, he forced you to either land in front of him or on the platform if you came down with an aerial.
7:14: You jumped again without a real plan. Going for the platform is ok, but if you're doing it, waveland and get off ASAP. I think staying on the ground and going for a grab/dtilt mixup, or just straight dashing through him, would have been better.
7:32: I might stop pointing these out, but again, jumping for no reason. That jump ended up costing you a whole stock.
8:07: Perfect. This is the first time you really went to punish his spot dodge.
Couple random notes here, just remember that your goal is never percent as Marth, it's positioning. You want to, ideally, get them off stage, or if you can't do that, get them above you. There were a handful of times where you gave Zutnop exactly what he wanted by jumping, and you got punished for it. Keep your feet on the ground as much as possible. Also, when getting up from the ledge, chances are you're gonna die anyways, but mix in neutral getup, ledge jump (the really high one, not dropping and jumping off the ledge), and wavelanding on/hax stalling. If you can do the hax stall, you can also mix in side B stall (just drop with back and immediately side B towards the stage) since it refreshes your side B. One of my favorite mixups against Marth is jump up > waveland off > immediately neutral stand up > grab/dtilt/ftilt. It's tough for Marth to punish because none of his moves are very meaty (which means they don't have many active frames) and, if you do it perfectly, you're only vulnerable for 2 frames at the very end of the stand up.
vs Kenny, I can't really help you here. I don't know anything at all about how to play Sheik vs Marth, from either side of the matchup. Hopefully someone else will talk about it here.
vs Zutnop again:
A lot of your issues here are the same as before, so I'm not gonna go through and specifically point every mistake out. I will say that, you played the neutral game a lot better this time around. But once you got a grab, you never got anything off of it. At real low percents, obviously fthrow chain grab or fthrow into fsmash are really good, at mid %s, up throw juggling is where it's at, whether with uairs or fairs or utilts. At real high %s, like 100%+, you can uthrow and go for a kill setup, but I like to just downthrow and try to tech chase with a tilt/fsmash/nair. Honestly though, don't sweat missing combos like that, just find this zutnop guy at the next tourney and play friendlies with him for a few hours and you'll learn how to combo Marth. One thing I will say is, try to tip all your aerials, I know that's easier said than done but it's something to think about while playing.
Random aside: I don't think you dash attacked once in any of the games. It's really strong in the ditto, so at the very least, you want to make them afraid that you COULD use it on them.
And finally, one thing that helps me a lot, especially in Marth dittos, is to always be consciously aware of whether I have the advantage, they have the advantage, or if it's neutral. If it's neutral, I tend to play things a bit safer, and when I have the advantage, I'll bust out wacky stuff every now and then to keep my opponent on their toes or mess with their DI.
Hope you find this useful. Honestly I think you just need some more experience in the matchup, even by the end of GFs you were doing a lot better with your punishes.