Here's a helpful bit of observation that popped up on reddit last night. A redditor called Caged put together the point differentials of NBA teams against above- and below-.500 opponents. Unsurprisingly, they look a little different from the straight-up version.
The chart above (tap "expand" or check out the original here) is the league sorted by point differential against above-.500 teams (blue bars) with the below-.500 differential in purple. The biggest difference you'll see is Dallas drops like a rock, falling from third in overall differential to 13th here. Only seven teams—Memphis, Toronto, Golden State, Portland, Cleveland, Houston, and San Antonio—have a positive differential against winning opponents, and only seven sad-ass teams have negative differential against losing teams.
The second chart is a little less instructive, but still interesting. It's the split between teams' differentials for winning and losing opponents. Instinct says to read this as the Mavs being fraudulent in some way, but that isn't necessarily the case. Good teams get blowout wins. So a team like the Mavs having blown out a bunch of opponents and struggled against good teams (they've lost to Miami, Portland, Houston, and San Antonio) early in the season could just be noise.
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