Learn Italian: Perché non ti è piaciuta questa decorazione?

With the verb piacere, the thing you like is actually the grammatical subject of the sentence. Here, the subject is questa decorazione (this decoration). Since it is a feminine singular noun, the past participle has to match it and end in -a.

Why do we use è instead of ha for the past tense here?

In the past tense (passato prossimo), the verb piacere always uses essere (to be) as its helping verb, never avere (to have). This is why we say è piaciuta instead of ha piaciuto.

If the English translation is 'didn't you like', why do we use ti?

Italian phrases with piacere are structured differently than English. Instead of 'you like the decoration', Italian essentially says 'the decoration is pleasing to you'. The word ti is an indirect object pronoun meaning 'to you'.

How do I know decorazione is a feminine noun?

Almost all Italian words ending in -zione, like decorazione, lezione, or stazione, are feminine. That is why it uses the feminine word for 'this' (questa) and triggers the feminine verb ending in piaciuta.