In theory/research papers, it's not a terrible idea, a few years ago, 3blue1brown on YouTube did a video about how blockchains and cryptocurrencies work at their core, setting the controversy aside. It is a currency that is actually tied to something of immutable valuable, time and logarithmic growth, and would require much more computational power than any singular entity could control over 50% of. Those goals are good goals.

The the irl problems come in on multiple aspects however, there are only so many graphics cards made every year, so people purchasing for mining impacted the entire market of graphics cards spiking prices higher and higher, they run on electricity which is still generated with a lot of fossils fuels, and cards that break because they're running all the time and are staying hot end often up in landfills, where lots of useful metals are just sitting and rusting away.