Esperanto miri comes from Romanian mirare "wonder" -> Latin mīrāre "wonder" -> mīrōr "astonished" -> mīrus "astonishing" -> PIE *sméyrōs "laughing, smiling" (same origin as English smile)
I changed the etymology from Italian to Romanian because it seemed to fit better.
Esperanto doesn't quite have the same usages as English. In English it would be understood that you are describing something when you say "Amazing!" and therefore you would use an adjective. In Esperanto, unless you actually specify the noun that you are describing, you seem to use an adverb. I speak as a beginner and could easily be wrong.
If we don't know that other sentence it is difficult to start a meaningful discussion. It could be that this other sentence has a subject, such as “(everything in) Canada is amazing;” in this case the translation would need to use the adjective, mirind
But without the sentence this is futile guesswork.