Of course its about the $ value
How would you feel about ROI on miner hardware if the reverse was true?
If you had paid 100 BTC for a miner when BTC was worth $100
You mined 101 BTC but BTC value dropped to $50
Would you say you have made a ROI?
Yes! More BTC than you started with is a profit. Less BTC than you started with is a loss.