Irresponsible
Pointless, Unfounded, Careless
な adjective
You add and you subtract. This brings you back to zero. There was absolutely no point in doing those actions. Completely pointless. In fact, I'd say that was downright irresponsible of you. The extra いい is there to mean "good," as in "good, you've figured out how pointless that was, now stop."
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
あんないい加減な人、見たことありません。
I have never seen such a lax person.
そういういい加減な情報を流すの、いい加減やめろよ！
You need to stop giving out that kind of lousy information!
このサービスはいい加減すぎます。いい加減、改善すべきですよ。
This service is clunky. It's about time for some improvements.