>Would it be a mistake to use Desmos in a math classroom

Maybe. Back in the day I had classes where we had to learn the rough shape of a number of basic functions, which built intuition that helped. This involved drawing a lot of them by hand. Initially by calculating points and estimating, and later by being given an arbitrary function and graphing it. Using Desmos too early would've prevented building these skills.

Once the skills are built, using it doesn't seem a major negative.

I think of it like a calculator. Don't let kids learning basic arithmetic to use a 4 function calculator, but once you hit algebra, that's find (but graphing calculators still aren't).

Best might be to mix it up, some with and some without, but no calculator is preferable to always calculator.