Well its a good question to ask, but remember that new laws governing stablecoins (ie. GENIUS act) mandates a 1:1 backing with US Treasuries.

So the USDT that are coming into creation are ultimately flowing into treasury purchases. In theory this isn't representing a new creation of money, but instead its just swapping hands something like: Existing USD -> Tether -> US Gov.

The interest portion of the US Treasuries created "in theory" has a deflationary effect.

Really this entire question revolves around Full-Reserve vs Fractional Reserve banking.....we have been under a regime of Frac Reserve for the past 100 or so years. Clearly Frac Reserve is inflationary (ie. money multiplier effect from relatively low reserve ratios). In "theory" Full Reserve banking should not be inflationary.