My internet is acting screwy so I can't watch the second at the moment, I'll add to this post when I can with an edit.
EDIT: Watched it
You need to be focusing on upthrowing him quickly on the opening grab or f-throwing once then regrabbing and up-throwing if he was at 0 due to how easily he can break it then, you don't want to knee him as you hold him until hes over 5%. Also, you up-tilted on the starting chaingrab way too early but at least you kept the combo going.
Once again, like in the other, you ran to the edge and dash attacked over wavedashing to it and f-smashing, which would have taken that stock..use your wavedash.
At higher damages, such as he was at 64%...start combos from the up-throw, don't try to land an f-smash, take to the air and f-air combo off of it. I'd start taking to the air from an up-throw if I grab him after 45%.
After about a minute, you rolled abit too much and seemed too timid about going on the offensive. You need to learn the right spacing so you know when you can attack him..both of you stayed at a distance for awhile, wavedashing toward him out of a dash dance would have been nice... And yes, as you said, your up-b cost you in this too.
You did decent with your chaingrabbing and punishing after you respawned and took his next stock, but shame on you for the pause.
You rolled too much for no reason after he respawned and it caused you to be predictable, thus him catching you and comboing.
Once again..stop getting up from the ledge into easily seen charged smash attacks.
You up-tilted again on your chaingrab far too early. Learn to smash di those upairs.
You didn't defend yourself when fox shined you on this stock, you seen him coming and didn't fair or anything..don't be so vunerable.
He firefoxed for god knows what reason and you didn't punish it..you should be able to capitalize on him doing things like that.
He comboed you for abit but you managed to grab..and you didn't chaingrab, you tried up-tilting again at far too low of a damage once again.
Your edgeguarding game against fox needs work..the counter was a bad idea, you should have realized the way he was coming from bottom straight up, he wouldn't have reached you at all.
At about 4.08, you threw away an easy edgeguarding attempt like you did in the first match by running offstage...then at 4.18, you backed off for no reason as he was firefoxing..you aren't making use of this situations.
You seem to be making the same mistakes so with abit of correction, your game will improve quite a bit. Learn the %s of chaingrabbing fox, be more defensive about your returns. Avoid having your back to the ledge with fox infront of you. Learn to smash di those upairs and be more thoughtful about the situation. Don't let the fox live when he makes mistakes like firefoxing on stage.
Also,now that I think of it, get different stage experience, fighting only at one or two stages limits your ability to react to the others.