How much should an U8 care about winning?

Wanting to win is completely normal. I would not try to remove it. The useful distinction is between children caring about the game and adults making the result the measure of whether the day was successful. Let her celebrate winning and feel disappointed when she loses. Then help her move on. You can ask about something she enjoyed, something she tried or what she wants to do next time without pretending the score did not exist. At U8, lots of short experiences of winning and losing can actually be healthy if the adults keep the emotional temperature sensible. Competition becomes unhelpful when selection, praise or mood changes dramatically because of the result. So yes, let them want to win. Just make sure development, enjoyment and effort are still valued when they do not.