Until high levels, counterpicking characters is detrimental; it might improve your results in the short term, but it ultimately leads to you not learning how to play your main properly. You need to learn how to adapt and deal with matchups, because counterpicking characters (especially garbage-tier tier characters like DK) to get around having to learn to play the game is unsustainable.
I'm not saying you have to pick a top tier (though you should), but whichever character you play, having one main and sticking to it is best for you as a player.
Jiggs, Marth - My doc does 200 percent better then my falco in these match ups.
That's your Doc, not Doc in general; they're still losing matchups for Doc. If you learn the matchup with Falco then your Falco will do better.
Mid Tier - All general toss ups
Falco - Ganon
Doc - Pikachu, Samus
DK - Doc, Luigi, Mario
Falco has better matchups with the characters you're using Doc for.
DK loses all of the matchups you're using him for; Luigi the worst of the 3. Since DK unequivocally loses to every remotely decent character, that's a given. Falco wrecks all of those characters.
DK - Sheik, Falcon, Ice Climbers - I was having issues with both my mains against these 3 characters, DK has been my low tier for awhile and recently I started to lab him against top tiers and realized that I do better against these characters with DK. DK has crazy strings on falcon and amazing gimp coverage. I still need to lab more against Sheik and Ice climbers but I find that the only way that my falco does better in these match ups is if I camp and I hate camping with a passion.
Sheik (especially NTSC Sheik), Ice Climbers, and Falcon are three of DK's worst matchups (and pretty easy for the opponent to play without matchup experience; spacie matchups are similarly difficult but it's easier for the opponent to be caught out be matchup inexperience). When talking about low tiers, "worst matchups" generally means "unwinnable matchups", and that holds true for DK. If you're counterpicking a character, then you should not counterpick one that basically auto-loses the matchup; even with the advantage of being gimmicky it's just a terrible idea.
Good luck being aggro with DK in any of those matchups.