In this personality test, the results are divided this way:
Introvert (I) or Extravert (E)
Intuitive (N) or Sensing (S)
Thinking (T) or Feeling (F)
Judging (J) or Perceiving (P)
Yea, it's true that she didn't appear often. But in these few scenes where we see interacting, she has a stable personality, she reacts the same way everytimes. And she seems to fit this personality well. But ofc they could have found someone better.
Another thing is... well, INTP in any fiction are quite difficult to make, since they literally never do anything. But the producers somehow managed to find a use for a character who almost never moves from its house. And it's Zecora, she never jumps into action. Meanwhile, Twilight tends to panic all the time, so she can't be INTP. So this chart above is the most accurate.