From which perspective - performance or financial? From a performance standpoint, I’d say clearly not a mistake. From a financial perspective, probably not that wise. (I’m not going to say “mistake” because they did it intentionally.)
But the big difference is, Machado was an established star, whereas Mullins is two years removed from being optioned to Bowie because he couldn't hit. Obviously Mullins has value because he is cheap and controllable, and hopefully the swtich to hitting left handed has yielded sustainable results, but Diaz (who was very highly ranked at the time), plus Kremer, Pop, and Bannon, is a deal you take for Mullins now.
We don't know where the 6 to 7 years before free agency ranks on the money list. It could be:
1. Shares of the national TV revenue?
2. Expansion dollars - How much new owners would play for club and what share the players get?
3. Streaming rights shares by the players?
4. Minor league TV right now that MLB has control of the minor leagues
5. And many other things.