Yes ammonia can safely be poured down a drain.
However never mix ammonia and bleach ever because it creates a deadly gas that can kill you very quickly when breathing it in.
But you can pour ammonia down a drain without any issues.
If you have a clogged drain you can pour some baking soda down the drain and then after the baking soda pour some undiluted vinegar into the drain and put the drain plug in and allow the vinegar and baking soda to fizz for around 20 minutes.
Then pour boiling hot water down the drain and it will usually clear the clog out.
Ammonia can help clean the drain as well but you need to also pour boiling water down the drain to clear the ammonia out and then run some hot water down the drain as well.