The difference between ent and undiscovered country is about 150 years isn't it. in TOS the companion cockran is found and said to have been lost over 100 years i think. Any way thats more than enough time for a civilisation that fights, and doesn't hold scientists in great respect(ent flox's conversations when trying to find the plague cure on Qo'onos) to ruin its own ecology.
Can you imagine someone standing in front of the klingon council and saying"we must protect the enviroment. Stop spewing smoke from the shipbuilders foundries, no more cutting down trees for fires and digging for metal ore for ba'atlaths"
Besides, i find it hard to belive that the scientists couldn't find some way to sort out the problem. TNG did in When the bough breaks. Didn't they reseed the ozone layer on Aldea? There's only 70 odd years between Tos and Tng so it should be possible to retrieve the planet