Only if the wildlife you shoot is tame and/or slow.
I shoot grizzly bears, bighorn rams and other wild animals. The crop IQ just lags behind. The center point on the 6D will get the job done.
High burst rate and solid tracking AF are really important for a ton of wildlife shooting, not just small birds. I would absolutely rather have a 7DII over a 6D for wildlife shooting because I would rather have sharp shots of peak moments with slightly worse IQ than soft shots of missed moments with slightly better IQ.
For me, lower noise and overall aesthetic quality supersede a few extra fps. Also, most of the prime wildlife is crepuscular, and in those conditions crop cameras have serious trouble.
I have the 5D III and 6D side by side with 70D (and formerly the 7D). I would only use the crops over the FF for tiny birds. Fur and feather looks too rough, even at lower ISO.