A lot of confounding variables, but there may actually be something here.
If you look at current R's and control for weather and the total number of people infected as of ~2 weeks ago (states that had bad outbreaks before are having fewer problems now), then you do see some apparent effects of mask policies. 
It may not be causal; mask policy may be a proxy for lots of other decisions that states take, i.e. their overall level of caution. Still, this is a stronger relationship than a lot of stuff I've looked at (mobility, restaurant reservations, partisanship, etc.)
Notes: in the regression above, I'm specifying mask policy on a 4-point scale based on the chart in the first tweet. If you take the results at face value (which, maybe you shouldn't!) a strict mask policy would reduce R by around ~15% vs. no mask policy.
