Do you write better when you are feeling sad?

It all depends, actually, on the way you approach writing. I would assume you are talking about fiction. All of us do reflect our mood swings and frame of mind (and heart) while writing. If you are prone to write in the manner that Thomas Hardy did (naturalism and its destined hazards), sadness will certainly filter in. However, it may or may not make your writing better.

Not at all times.  But sometimes, emotions can trigger me to write a piece.

Not always, but in my case alone, if its about sad times, the sadder I am the better I am able to express myself on paper.

You write what you feel right . Emotions do play a role & specially when it is to write - but true human is one who feels the same ( in all moments of natural change in human ) . He/ She has to have the control on emotions -to write truth & truth is always bitter . It can never be sweet .