Diddy vs Luigi isn't actually too bad now for Diddy. Or at least, I personally know how to deal with the matchup better than before. After the patch, it became noticable that Luigi had a better air game than Diddy. And due to his traction, Luigi reacts differently to bananas than most characters. However, Diddy isn't as easily out-camped like most other characters by Luigi due to his mobility. U Air also still kills Luigi at a respectable ~125% or so. I have much more trouble killing off Mario than Luigi because of this. Peanuts are a nice tool vs Luigi, but fireballs stop them completely so they are not always as reliable. Best is to try and hit Luigi in the air with peanuts so they hop over his fireballs. Otherwise Side B should take care of a spammy Weegee.
I don't think the matchup is really bad actually. At worst 55-45 in Luigi's favor. It's definitely not 60-40. Am personally leaning towards 50-50.
Also, Exploding Rocket Barrel is fun <3 think this custom is really underrated for Diddy. It gives him a solid attacking move and it charges faster than regular Up B.
I think I have to disagree a little. You ARE out-camped due to Fireballs. And if you try to Monkey Flip through, Luigi can duck, I repeat, DUCK (According to one Luigi main who implies this is true), and JC U-smash you (If that Luigi is a pro, he probably can just run after you and punish with a grab). If you try to peanut in the air, you seem to be exposing yourself too much for a grab/Cyclone/whatever when you land.
I too don't think the MU is bad (Not 3:7 bad. And even 4:6 a stretch). Maybe it's 55:45 like you said, maybe even 50:50, but some Luigi mains seem to agree this is 55:45 in Luigi's favor.
Custom is honestly underrated for half of characters. Most people know Mario, Palutena, and DK gains a lot from customs. But I DO think Luigi gains a lot too.
Quick Missile buffs his recovery that an uncharged version travels about as far as a fully charged Green Missile (Albeit it took forever to act out from, but unlike the regular Missile this one keeps going with respectable speed, so it's not as gimpable).
Floating Missile goes straight and slow, but it charges up quickly and has the least ending lag of all Missiles, and the ending stalls Luigi in the air. Though this is definitely gimpable.
Iceball has its perks (Freezing people instead of causing almost no hitstun, so it's not punishable on hit, and it lingers and stays for like forever).
Burial Header buffs Luigi's recovery and, while it removes one of his deadliest punish move, the burying helps Luigi to setups some nasty things (A free smash at mid percents, and a smash deals 14-15%, you can even charge them if time allows, allowing for guaranteed damage outside of down throw.
Cyclone customs aren't really worth trading over the original though.
But sadly, Luigi isn't on the train of the blasted 'Super Windbox Bros.'. I mean, come on!