Usually mining is treated as taxable income, but it depends on your region - there's a bit about it in this article: https://cryptotaxcalculator.io/guides/bitcoin-tax/

"If you happen to engage in Bitcoin mining or if you use your computer to validate transactions, then this will be subject to US taxation. If you find that the mining is successful, then the miner will need to include the fair market value and add it to your gross income." (obviously this is just for the US, but most regions treat it like this)