https://www.youtube.com/watch?v=hC7BAh2sjEc&t=2m20s
Got bopped, wanna know why. I'm not doing something right in neutral.
Honestly dude, I'm not sure it was completely your neutral. Game 1, you punishes were lacking. Game 2, you counter-picked yourself on stadium. Game 4 you were on tilt, you SD'd, and just had some pretty bad DI that got you combed and dtilted or daired for every stock.
Game 1 your neutral was fine. It was a close game, and like I said, your punishes were a bit lackluster. Both the raptor boosts that you hit on the first stock didn't net you much. At 3:30 you get a two grabs and you up throw into a trade both times, meaning you need to practice your speed after those. At 4:00 you get a stomp and miss the tech-chase on the top platform, which is hard, but would have been nice to hit. You down-threw into nothing a few times, and you didn't have a very strong presence on the ledge. I thought you should have grabbed ledge or invincibly planked more often to force the marth on stage for a knee, but you let him DJ to ledge quite a bit which is sort of a free recovery. You also started the Marth slayer sometimes but let it go or just did it when they were close enough to DJ to ledge. Force them low first, or don;t be afraid to fall to ledge ad then knee them.
That being said for game 1, your platform movement was spectacular, and your apparent comfort on them fun to watch. your movement on them and around the stage was tricky and was what I thought allowed you to keep up with the Marth for the most part. I thought it was a cool choice when you tried to set up platform tech-chases pretty often.
That platform movement is part of why I thought you counter picked yourself game 2. Fisrt of all, stadium's biggest benefit is its increased space for dash-dancing, and with your platform-heavy style of movement, you didn't take much advantage of the extra space, it just became more space for Marth to control. Often times you did a single dash back into running Nair forward, which isn't really making use of the space. That sort of mistake is part of the neutral, but I think you felt cornered a lot of the time without bigger platforms to play with and escape to. BF's platforms make up a large portion of it's horizontal surface area, which is not the case on PS. The smaller length of the platforms made them easier for Marth to cover, and gave you less opportunity to vie for stage control from them. Also the lack of a top platform gave you less space to escape to when Marth had control of center stage. In game 1 you used it several times as a place to jump up to and fall down as a way to push Marth out of the center, or to escape a combo, and you also recovered to it once (and it was a sick recovery by the way). Without that, I really think you limited yourself based on your style in game 1.
Game 4:
13:50--- DI this out, this is what made you lose the first stock. After that it just looks like you're on tilt. You;re not moving as well as game 1, you drop an edge-guard, and you SD the second stock. 14:46---- you mis-space your DJ and get D-tilted, that should have been death again. You panick Raptor boost after this. The trouble with Yoshi's is that it's just so easy for Marth to get you off-stage with F-throw tech-chases and then dominate Falcon on the edge and it's just so demoralizing. I don't want to talk that much more about game 4 just because I don't think it's a good representation of your playing ability. Also marth can mess up your platform movement here super easily, since up tilt covers an entire platform. You can't really come down on him anymore, or DD on platforms, they're small enough that he'll just catch you and then edge-guard you, so it's another style counter pick stage for you.
As far as actual advice on the neutral, I think you need to be a little more ground based. marth likes ground control, and usually he doesn't actually mind if you're DDing on the opposite side platform. I think you should try dashing around right at the edge of Marth's JC grab or fair range. This puts more pressure on him and the threat of you invading his space more than standard platform movement does because of how easy it is for you to simply grab him if he makes a slight spacing mistake. Dashing right around that range makes it harder for him to react to your Nairs should you decide to approach him. He won't have as much time to react with an up tilt as he might if you did the Nair from across the stage or from on a platform.