Good design is when you create great added value for both the producer and the consumer.

Onur Cobanli ཡིས་འགྲེལ་བཤད་བརྗོད་པ།
For producer, good design means products that cost less to produce (such as thanks to better production planning, less waste creation, use of correct materials and technologies that lead to cheaper production, transportation ease, stack-able, durable etc), less to sell (aesthetic and looking good, thus easier to be desired, self marketing, self promoting). For consumers, all the same things; but from the other side: sustainable, desirable, durable, functional. Good design makes everyone happy, it creates utility and added value for all parties, not just the buyer nor just the seller, thus good design is sustainable by its definition.