No big revelation here, just writing this as a reminder to myself next time I try curl localhost from WSL and it doesn't work:

1. Don't use localhost or 127.0.0.1. Use your machine IP. But you shouldn't even need to figure this out. Using your-computer-name>.local should work (eg curl Joshs-PC.local. Even better: curl $(hostname).local.
2. Make sure your server is listening on 0.0.0.0, not 127.0.0.1.

Source: this excellent StackOverflow answer.