In general they've done better selling at the deadline than they have buying at the deadline. Part of that is inherent to deadline deal trades. The buyer is going to pay significantly above 'retail' prices at the end of July.
Trades in the offseason are generally where you'll see even value. Nobody brings up the Josh Donaldson deal where the Twins absolutely destroyed the Yankees. Trading away oft-injured Mitch Garver looks like a win and it set up the Donaldson deal. The Arraez trade will be won or lost based on whether Salas and Chourio develop. The Farmer and Taylor deals look good. I wouldn't say they've done "extremely poorly". I think they've won about as many trades as they have lost and several of the "lost" trades are due to injuries.
I'd give this front office a C grade for trades. My bigger issue is they don't acquire enough international free agent pitching talent. They have to be one of the worst organizations for identifying, signing and developing Latin pitchers.