I like 'em too. They didn't look out of place and just because they had only one plot function, aside from the "set decoration" purpose of inhabiting an alien planet, I don't think they were too contrived. How many times in stories have protagonists followed animals to escape an enclosed space? I don't think these crystal foxes were a bad idea or badly used, is what I'm saying. That whacking great asteroid dwelling worm thing in TESB had only one function as well and exists now as an iconic scene that nobody complains about. These beautiful crystal critters will probably never be as iconic but I'm glad they're there and I have no problem with their role in the narrative. They're certainly a better, more "Star Warsey" idea than somebody lighting a torch and following the smoke.