^
2 huge things i noticed.
- when you're in the air, you'll do one of two moves. you'll fair or you'll bair. every time you bair when he is on the ground you get punished, because that move isn't safe on shield. you were more successful with fair, however it is slow and you did get tornado'd for it a few times. you need to use all your aerials when he's in the air with you. you also got faired alot and if you retreated your fair instead of going into him with it you would've been fine and you would've hit him out of his. also, you should use SH dair. it's great for punishing him when he runs in (which he does alot). if you SH dair to float nair you can usually catch him when he runs in, because he either wont expect the dair or he'll not have run in, expect to punish the dair, and get hit with nair. dont do this continually. but anytime he runs in on the ground you should be punishing that. you can just dash attack him or jab him as well.
- the other thing is that you're way too grounded. i know people say to stay grounded vs. mk but you'll never win like that because he isn't vulnerable on the ground and peach's ground game isn't good enough. i'm not saying dont use her ground game but all he has to do is run up and shield. her grab is trash so she can't threaten him with that unless he stupidly gets that close (which he doenst have to do). once he runs up and shields the ftit, jab, whiffed grab, fsmash, turnip, he's in close enough range to do a shuttle loop and it will hit you regardless of whether it looks like it should or not because mk is ******** and so were the programmers of this game. anyways, when he's in the air (in front of you or above you) you can actually hit him. he has no shield and will be trying to hit you with a move cause he thinks he can. this is the part where you do some retreating aerials (esp. fair). you can outspace everything he does in the air and there's nothing he can do about it. if you bring turnips along with you it makes combating him even better.
your defense game is fine, but your offense game (against mk anyway) could use some tweaking. hope that helps!