Capn Proto and QUIC are are layer 6 and 7 (presentation and application protocols respectively). Quic is built on top of UDP.

Layers 3-4 (network, transport) are both big-endian - IP packet headers and TCP/UDP headers use big-endian format.

This means you can't have an IP stack (let alone TCP/UDP, Quic, Capn Proto) that's little-endian all the way through without breaking the internet.

Outside the webdev bubble, it's pretty much QUIC that is irrelevant - it's just another UDP based application protocol.