DK vs Rosa is obviously a bad MU for DK, but I wouldn't go so far as to say she hard counters him. I would say she is a soft counter, so I'd have to go with +1.5 (or w/e 65:35 would be) Rosa in the default meta. With customs, DK has a way easier time and has access to much better anti-Luma tools. Overall with customs, I'd say it's +0 at worst for DK.
Most of what makes this a bad MU for DK is just how much random jank Luma is capable of doing. DK doesn't have that many attacks with enough base knockback to knock Luma away without prior damage, so he is prone to random Luma jank counter hits. Luma also gets to counter hit DK for free while getting carried during his Cargo Hold, tripped by Dtilt, or dizzy from Side B shield breaks. All of DK's Cargo Combos are basically sealed away while Luma is nearby. DK's Hand Slap won't hit Luma since it floats too high in the air 99% of the time. Rage Luma KO's DK absurdly early with thanks to it's insanely high base knockback, effectively negating most of DK's natural bulk. Luma rapid jab loop while DK is on the ledge is pretty much free damage.
What keeps DK from losing too horribly is Rosa being floaty, lightweight, tall, and having an exploitable recovery. Her slow fall speed makes her landings easier to abuse and anticipate. Her weight + low gravity makes her vulnerable to pretty early KO's, especially vertical KO moves like Dsmash or DK's infamous grab combo. Spacing SH Bair against Rosa is easier than against most others due to how tall she is and rising FH Bair will hit her as well. DK is pretty good at edgeguarding, so recoveries that don't have hitboxes are pretty easy for him to intercept. Rosa has enough distance, mobility, and range of motion to mix it up and avoid getting spiked most of the time, but DK can safely rack up damage or go for stage spikes with his lingering aerials. DK can also exploit Luma to hit Rosa when she dodges or gets up from the ledge by having the hitlag from hitting Luma prolong the attack until Rosa's invincibility wears off.
With customs on, even though Rosalina improves with Shooting Star Bit and Luma Warp, DK gains way more. With Kong Cyclone, he can actually land and easily challenge Rosa in the air at any time. Jumping Headbutt gives DK a juggle interrupting/landing mixup that also knocks Luma away, unlike the default. Hot Slap excels at shutting down airborne opponents, including Luma, and doubles as a legit KO move. Lightning Punch gives DK a reliable safe on block poke that knocks Luma away. Storm Punch can gimp or outright KO Rosa when done at the right time and puts her in a bad position offstage when it doesn't. Luma just gets so easy to slap around with customs that the MU improves dramatically, despite Rosa's better specials. After all, without Luma the only special move she can use is Up B lol.
If DK manages to get some momentum by getting rid of Luma efficiently (customs really help here) or landing an early KO move, DK has a good chance to take the match. Otherwise, it is an uphill battle of attrition that DK will lose more often than not.
