Ouch, lots of dash attacks, Praxis, your diddy looks like mine week1, but not bad
What you will come to learn is Diddy is about being unpredictable, that you have to be very versitile, otherwise you'll lose easily... Like getting the AT's down is one thing {I suggest you 1st practice Glidetossing which you pickd up fast, Z Catching, and the Stutter Step Fsmash [great for ending a banana lock] but also learn others, but focus on that 1st... check out the Diddy Kondpedium or whatever}, but being able to incorperate them with like mindgames and such is the hard part...
I'll start with your DSF (MK) Match [I'm not gonna critique the luigi match cuz luigi is one of diddys matchups where you have to change your playstyle a bit]
Off the start you stood still, pivoited {cant spell} and down-b for your nana... BAD... Cuz 90% of the time when you play any1 off the start they WILL rush towards you and beat you to your banana, leaving you helpless... The most common/safe way is to full jump off edge {or near edge}, and down b, and jump again to Z catch it... ( {also as a quick note sometimes its good to not even take out a banana off the start and instead wait for the rush and Side-B hump them or whatever}
Second is that you tried fighting with only one banana multiple times, which is bad because well why have one when you can have 2, 2 nanas give you much more options and with them you need to practice controlling them... now back to being predictable... your recovery needs some work, but i'd like to point out that you tend to just Dash attack into a banana then throw it like a second later.... sometimes its good to keep hold of your banana maybe keep it in hand to down b for another [very useful cuz u can throw the one in hand if oponent rushes], Side-B kicks/humps ... or just overall mindgames [dribbliing to fake a throw and doing attacks]... the #1 best thing to do with a banana in hand is PEANUTS... r amazing, cuz it limits your opnent to camp, go under them and get nana'd or jump over and get Glidetoss [throw up obviously
] Nana'd into a F-Air or Uair... I saw you do that a couple times which was good but practice it more [u normaly did when he was way above u, somtimes its good to do is as they r trying to land]
The number one problem i saw though is u like to Smash Down-B like in the middle of FD and fullhop to get it, Diddy is ground-based... and going up there to get ur nana'r against mk, i saw u get punished... and there were a couple instances where you had a possible nana'r lock and al u needed to do was shorthop and Z-catch or instantthrow [which u havent learned yet, id stick to Zcatch for now] and follow up...
as for your dash attack problem, heres ur solution...
1. Dash, let go of foward... slide, and hit A as u almost stop on it...
2. Walk more... and just hit A...
3. Dash, let go of foward... slide, hold down and Grab
Problem solved
oh and learn the JCT, very useful in some matchups...
and with the downsmash issue, nana lock>Edge and Dsmash... works ALOT
Hope this helps...