“Mug” could mean many things. Blue ceramic mug tells a shopper more about the color, material and product.
That is the idea behind a long-tail keyword: a more specific search.
Build a phrase from your product
Write down what it is, what it's made of and a useful detail. Try the phrase in Keyword Search.
Look at the Long Tail indicator and the products shown. If you sell finished mugs and the results are mostly digital designs, adjust the words.
Compare it with a broader phrase
Put ceramic mug and blue ceramic mug into Compare. The specific phrase may have fewer searches and still fit your item well.
Adding more words doesn't automatically make a keyword better. Each word should help describe something you offer.
Save a useful phrase in a Keyword List. If it's too long for one Etsy tag, shorten it without losing the meaning.