"The mango has a good smell."
Translation:למנגו יש ריח טוב.
The smell of the mango is good = הריח של המנגו טוב. A copula is not necessary here, since it's <noun> is <adjective> type of sentence (so זה shouldn't be used). את doesn't belong here because there's no verb (or יש/אין/צריך).
Also, notice that it's a bit different from the sentence you were supposed to translate.