It was bad ending, but it wasn't as bad as many people make it out to be. We got a whole season of meeting the mother and it was great. The last episode, while it does undo a number of things, isn't really important to the overall story. If you just ignore it, or if it never happened, everything else still works.

The fact that you can and probably should ignore the finale still makes it a bad episode and a bad finale, but it doesn't retroactively make the rest of the series pointless and it doesn't answer any questions that weren't already answered.