IANAL but to my understanding its kind of a stretch to call it double-taxed.
Note: the below is assuming you're doing this mining activity as a private person and not setting it up as a separate legal entity like an LLC or something.
Simplifying, lets say 1BTC==$1USD at time of NiceHash payout, and you received 10 bitcoin. You'd then pay something like your regular income tax rate, lets say 30% on that. So the tax on mining income was $3. You'd still potentially have 10BTC, assuming you paid the tax with other dollars on hand instead of instantly selling.
Then crypto goes up from 1BTC==$1 to 1BTC==$1.50. Your crypto is now worth 10 * 1.5 = $15USD. When you go to sell, you don't pay tax on the full $15 you just pay the tax on the gains, $5. So you'd pay the capital gains tax, probably 15% to you, on that $5 so $0.75USD.
You don't pay the capital gains tax rate on the entire value of your crypto, only the gain in value. So from a USD perspective you didn't get double taxed. You had an income of $10, then you had an "income" of $5 when you realized the gains. That's two different taxable events, not a single one. If you exchanged the BTC to USD immediately at the distribution, you wouldn't of had a capital gains tax as you wouldn't have theoretically experienced any gains/losses on that distribution.
IANAL, this is not tax advice, I am entirely a lay person. If I'm wrong please correct me.