We can look at it this way: You're getting a D in Spanish and a B in math. To keep things even, assume both subjects are equally useful to your future. Do you spend more time to raise the D to a C, or to raise your B to an A?
Or if you have all Cs, are you going to try to raise one to a B, or try to pick up a new thing that you can learn to do at a C level? Or if you plan to go into STEM and want to drop Spanish, then raising that B to an A is probably better for you. Or if you have an A already, do you want to raise it to an A+, or add something else?
If you're just concerned with your GPA, then raising your bad grades will help. But if you're concerned with an accumulation of knowledge, then learning more skills or rounding out your weaker skills will probably help more.
Applying it to martial arts:
Within an art, it's probably best to raise your worst grades. To prioritize eliminating bad habits over learning more complicated or niche techniques. However, sometimes the more complicated technique can teach you more about the basic version. It takes more knowledge of a roundhouse kick in order to do a tornado kick. Doing a tornado kick can help you better understand the roundhouse kick.
Overall, it depends on your goals. I consider myself very good at TKD. Continuing TKD training will lead to very small gains in my ability. In fact, if my goal was to compete, I think Yoga, weight training, and running would probably give me bigger gains in TKD than TKD class itself at this point. However, if you give me a few months in another art (such as BJJ), then I think I will learn a ton of new skills in a very short amount of time.