The main purpose of stablecoins is not to act like a store of value,but to make cryptocurrency trading easier and more convenient.The stablecoins aren't competitors of Bitcoin and the big altcoins.They are supposed to be a supplement of the cryptocurrency trading ecosystem.
It depends on what we called stable coin, the ones that are backed by gold and have told price and the ones that are backed by bitcoin and have bitcoin price are can be said to be deflationary. They are also stable coins because their price is stable with the price of the asset they are backed with just like wBTC that is pegged with bitcoin or Paxgold that is pegged with gold just in my previous explanation. But about the ones that people most commonly call stable coin because they are prefer with fiat, I can assume that they are inflationary but not directly but indirectly as the Fmfiat they are backed with are declining in price.