英文口試時應該學唐英年講"完全是垃圾"嗎?

I would not recommend it. You would give the impression that you are a copy-cat.The comment itself is quite extreme and rude and would definitely be so in an oral exam .
When faced with a similar situation, I suggest you ask the speaker to clarify what he or she means by 'good' and what about Henry Tang he or she is referring to.
If you must respond by that comment, be prepared to elaborate why you feel so. You must get your points across clearly and convincingly, and to show how good your English is. After all, it is a test where you want to score high marks.

唔好啦, 呢句就算唔系粗口, 都唔系太文雅.
甘樣會令考官覺得你, 講野唔文雅, 甚至粗鄙.
而且各人有各人既體法, 唔好甘樣講啦!