Do stablecoins count as DeFi? Pretty much, if you ask me.
Stablecoins like Tether obviously aren't. They are centrally issued, redeemed, and controlled. Tether can freeze and blacklist USDT, etc.
DAI is a lot more decentralized, but not fully, and its centralized points of failure were on full display during the March 2020 crash and its aftermath. In truth DAI is mostly governed as an oligarchy by MKR whales, it can't maintain its dollar peg during extreme crypto volatility, and its collateral backing is now almost half non-bearer assets (including centralized stablecoins like USDC).
https://blog.makerdao.com/the-market-collapse-of-march-12-2020-how-it-impacted-makerdao/
I would say stablecoins can be integrated into Defi very easily since they can be easily tokenized on protocols like Ethereum, but that still doesn't make them decentralized.