Negative feedback is toughest to receive when it’s vague, and obscure.
Remember that when you tell someone you don’t like something they’ve done, or worst, something they’ve made. When someone creates something, and has the courage (because it does take courage to put yourself up for judgment), to show you their work, they’re trusting you to tell them what’s on your mind.
It’s okay not to say anything positive. It’s perfectly fair for you to say “no, that’s not quite right”, or “woah, you took all that time to make this?”, and it’s valid for you to not enjoy something made by someone.
Just remember to articulate it a bit better than “I didn’t like it”. Here’s why. Anyone who creates will want to know how to make things that you appreciate. Everyone has subjective views on things, so someone creating exclusively for you, would like to know what works for you, and what your tastes are like.
The only way for them to find out is if you tell them why you didn’t like something.
Take the other scenario. What if they’re creating for the public? Maybe you’re not the greatest critic, or the leading expert in the field they’re creating in. But, opinions always help. Someone’s more likely to work on finer details if you explain to them that what they’ve done doesn’t sit right with you for a particular reason.
Unreasoned, negative feedback, has to be, hands-down, the most depressing thing I’ve seen.