It should be the opposite. Crypto mining only makes sense if you’re getting a good deal on electricity, which means they’re probably running on off-peak hours. What that does is encourage electricity generation to increase the base level supply, which should decrease brownouts.
Brownouts are caused by unexpected peak electricity demand or some kind of equipment malfunction (e.g. inclement weather). It’s not caused by a consistent increase in base level demand, which is what crypto mining would do.
Keeping fossil fuel plant open longer is a valid concern though, but since solar and other green energies tend to be cheaper over time, I don’t think that’s a significant concern. I could absolutely be wrong though, I don’t work in energy generation.