Is it OK for dishwasher waste to be connected lower than sink trap elbow?
I'm not sure anyone has answered your question yet. It's hard to tell from the picture but I'll defer judgement to the &quot;engineer&quot; on site that the drain line from the dishwasher is somewhat lower than the drain feed out to the sewer line. If that's the case this would cause some of the water (weir) in the trap to back up into the lower part of the dishwasher drain.
To put this in perspective - it's probably not going to cause you major issues. However, since it's under warranty - have them fix it. I'd recommend isherwood's solution above - have them shorten the leg above the dishwasher drain and the corresponding ell going out to the mainline.
The reason I recommend that is based on the picture. The vertical rise from the trap to the horizontal drain piece looks to be border-line long. To explain, here's an illustration of a proper setup:

The height of the weir (area between the red lines) should not be more than 4 inches. Again it's hard to tell but the weir on your trap looks higher than 4 inches which would make it less effective in flushing out debris.
Edit
If the weir height is no more than 4 inches the installers have the option of moving the dishwasher drain up to the higher unused fitting going into the tailpipe on the right or raising the trap as indicated by thye blue lines in the picture. As Harper mentioned in his comment, those pipes may be pre-fab and not adjustable so the installer may have to install new pipes above the trap to get the dishwasher drain line above the outlet pipe on the left.