So I'm having a really hard time practicing techskill,
shown here.It feels harder than when I do it on an actual gamecube, so I'm not sure if this is some sort of mental confirmation bias or if my
dolphin controller settings are off, since I'm not using a Mayflash adaptor (rather, I'm using a 3-in-1 GC/PS2/XBOX controller adaptor). So do I just suck or are my setting off?
No one else has any way to tell if the inputs we're watching are because of you or your setup. You should just get an actual Cube and controller and practice that way. Also try SHFFLing with an easier character first. If you are brand new to Smash in general, then it's certainly possible your setup is fine. All I can tell you is that you are FHing some of the time, not FFing some of the time, and inputting the aerial at the wrong time. If you have never SHFFLed before, you should break it down into parts to practice. First, practice SHing. Just press and quickly release the Y button (or whatever you're using) before your character leaves the ground (slower jumping characters like Ganon or Bowser are preferable to fast jumping ones like Fox, Samus, or Sheik). Once you can SH most of the time, start FFing each time. You can only input a FF once you've reached the peak of your jump, so as soon as your character peaks, press down quickly. Pretty simple, and in no time you will be bobbing up and down with SHFFs. Take the FF part, maybe even the SH part out, and just focus on jumping and aerialing. Depending on how comfortable you are, you can learn the aerial with L-cancelling or learn without and add it back in later.
As long as you incrementally add in each section like this, you will improve much more quickly than if you just try and spam all of them at once. Trying to just SHFFL outright is comparable to someone who has never played the piano trying to play a full song. Sure, with enough practice you can get used to the pattern and pressing the keys, but someone who learns the fundamentals of playing the piano first will grasp the song much better and perform it more accurately sooner.