Hi, Leonardo. "She got astonished" here means "She became astonished" (She got angry = She became angry.....etc), so it's active. However, I wouldn't use 'get' in this sentence, because you don't generally become astonished. It's not a process. Something happens and you're immediately surprised/astonished. I think the sentence would be better if you said "She WAS astonished when I told her the truth".

By the way, the passive is when we put the object first. Tom saw the cat (active) The cat was seen by Tom (passive) In your sentence (She got astonished when I told her the truth), there is no object.

We do use 'get' a lot for become, Leonardo, and it is informal - you're quite right about that. But it always has to be something that is a process - that takes time, and isn't immediate. You can get cold, get angry, get frustrated, etc., so it is definitely a good idea to use 'get' when you can. Good luck with the English learning.