Is this odometry considered bad?

edit

That doesn't look like terrible odometry as most of the lines seem straight. It looks like you ended up with an incorrect match at some point which pulled you out of alignment. If you had a ground truth it would be easier to compare. Typical odometry tests for odometry is to drive in a known patter and compare the output of a closed loop trajectory.

You may be able to tune the gmapping parameters better for your platform. I would also suggest doing a more incremental exploration and coming back to the area you started more often instead of making a large loop and trying to close it at once. Gmapping doesn't reoptimize the whole map only recent loop closures.