Yes, mountaintop removing coal mining is environmentally damaging as it takes the very top of the mountain's ecosystem away. This can have a myriad of unknown and unwanted effects on the local ecosystem. Needless to say, such is quite undesirable. When unknown damages occur, the entire ecosystem may be thrown out of whack.
It shouldn't take more than the name "mountaintop removal" to understand that act of removing the top of a mountain is damaging to the environment. But, the disgustic process is even worse than it sounds.
Rivers are corrupted, forests are destroyed, the air is polluted, and the mountain itself is just gone. All for some coal, which is one of the dirtiest and most polluting types of energy available to man. The process is destructive from beginning to end and does not provide benefits equal to the damage it causes.