That decoder needs the full LoRaWAN message, as seen on the gateway. That includes about 13 additional bytes for things like some header, the DevAddr and a MIC.
Your application’s Data page in TTN Console sees the application payload. For that, TTN does not know how it’s encoded by your device. But surely 0x48656C6C6F2C20776F726C6421 is Hello, world!.
(Next step: don’t send text.)