Is "you should not believe everything you hear or see before you experience it yourself" a good theme for "All Summer in a Day" by Ray Bradbury?

I would say this theme doesn't quite get at the heart of the story. The children are envious of Margot because she remembers having seen the sun and they don't: it rains all the time on Venus and they arrived on the planet when they were younger than Margot. But that really isn't the most important part of the story. What matters is what the children do in response to envying Margot for having witnessing something that they haven't. They can't, it is true, really know what the sun is like until they experience it for themselves, but they do believe it is truly a good thing before they see it: as the story opens, they are anxiously looking forward to the hour of sun that comes once every seven years. A better theme would be that we shouldn't bully or treat people cruelly because we envy them or that the children may feel more remorse over locking Margot up after they have experienced the sun, an experience so thrilling that it caused them to forget all about her.