To answer this question, let's first look at what it means to be perfect. For something to be perfect, it has to be free of all defects, absolute, and complete. Therefore, in order to obtain this status, you have to be free of any defects relative to our point of view as humans.
Your first question is on whether the statement is agreeable, and is therefore an opinion based question. In my view, the statement is true because how we practice marks what goal we achieve. If we put our full effort in, we maximize our output, and similarly, if we only practice half-heartedly, our achievement won't be as fulfilled. With this same ideology, perfection can only be achieved with practice that is marked with perfection.
Your second question asks about whether it is applicable to everything (outside of dance), and it should be fair to say that this is true. Practice can be applied to any goal in which we desire to better ourselves. Therefore, anything which we have a different array of skillfulness in can be improved upon with practice, so with perfect practice we can become perfect.
Now if we apply mediocracy to this train of thought, we can come to the conclusion that those who are mediocre in a certain skillset only arrived at that level due to their mediocre level of practice. Mediocrity, or the state of being at moderate quality, is a level such as perfection, so the level of practice can be applied to reach said level.