An interesting question. I'd start by suggesting that your "if and only if" is open to challenge. First the less important part for our purposes: the fact that somebody values something doesn't obviously mean it's actually valuable. Some people value terrible thing, after all. There's a chilling scene in James Clavell's novel Shogun in which a samurai takes deep, deliberate and despicable pleasure in the screams of a man being tortured to death by a torturer who specializes in optimal cruelty. The samurai clearly values the experience, but I'm not willing to say that it's therefore valuable.
Still, it seems right that value has a deep connection with experiences and the beings who have them. That may seem to be all that your worry needs. In other words it might seem that something is valuable only if someone values it or, more broadly, only if it evokes the right sort of reaction in some sentient being. But this is too strong. Consider: something could be exquisitely beautiful (to take one kind of value) even if no one ever encountered it; beauty unnoticed is beauty all the same. This doesn't mean that there's no connection between value and experience; it just means that a counterfactual connection can be enough. Crudely, we might say that something is beautiful if it would evoke the right sort of reaction in a sentient being, given the right circumstances -- whether or not it ever does.
All that is background to addressing your main question: is the appearance of life in the world a good thing? Here are a couple of thoughts. Some things are valuable because they have the capacity to produce certain kinds of experience. But some of those experiences are valuable for their own sake. And so the appearance of sentient creatures in the universe brings two things with it: the possibility for value to be "realized" and as part of that, appearance of a kind of value that a universe without life would lack.
Does that mean that the appearance of sentient life is an unmitigated good thing? Obviously not. Before sentient life arrived, there was no pain and no cruelty. We like to think that on balance, life is a good thing, but there's no necessity in that and it might not even be true.