It's light that makes someone look worse than they really do. It doesn't make them look good. Maybe it is too bright or from a bad angle. I hope this helps :)

Could you give us more context? If someone presents something in an unflattering light, it means that they present the information in a way that makes someone or something look bad. This can be literal as most of the comments below explain, or it can be in a way that represents something badly. But without knowing the exact context, it is hard to tell you which one it is.

I believe unflattering light is too dark or too bright so that it doesn't help a person's appearance. What do you think? Unflattering describes the light.