they can have pegged stable coin to pegged mcents
This can happen only if all the channels are opened using the colored bitcoin. This isn't the case with Lightning, but a sub-network could become a thing if needed, wherein sub-nodes route and make transactions with other sub-nodes, using the colored bitcoin as a unit (in this case, USDT).
Tether is also transferred using bitcoin blockchain called the omini layer. If you are talking about smart contract, then you are talking about DeFi, NFTs and staking.
No, I'm saying exactly what you're saying. It's a token based on another network's unit. It's colored bitcoin.