I think with DeFi it's more of the platform allowing financial activity without centralized entity intervention, although stablecoin is centralized but the dex won't freeze a tx because it's supposedly to be permissionless protocol.
So I think they're still defi despite using stablecoin as a trading pair for perps platform or still provide stablecoin swapping for dex in general.
On the other hand, the DeFi platform can always provide route for decentralized coin swapping to another decentralized swapping without stablecoin at all.
The DEX won't censor trades linked to stablecoins. But what use would that be if the stablecoin issuer itself can put you on a "blacklist"? Technically, you'd be excluded from participating in "De-Fi". For truly-decentralized finance, everything needs to be decentralized (starting from the tokens, all the way down to the core protocol). The problem is that most decentralized stablecoins aren't reliable as their centralized counterparts. Either they're algorithmic or collaterized by other cryptocurrencies. With no regulatory entity behind their operations, it would be the "wild west". Especially when there won't be investor protections or guarantees of any kind. That's the biggest downside of using decentralized stablecoins on "De-Fi".
The ideal solution would be a hybrid one ("CeDeFi"). But this will come at a cost of reduced privacy and freedom. I know many people will choose this route, since they prefer convenience above all else. Therefore, making true "De-Fi" rise in mainstream adoption would be nothing more than a pipe dream. Only time will tell...