It is bad to leave poop in the toilet.
Poop contains bacteria which can get into the air and make people sick and the poop will also smell bad and make the bathroom smell bad and the smell could go into the rest of the home.
Always flush the poop after you poop in the toilet to get rid of it.
Especially if you have others using the toilet in the same house they will not like it if you leave your poop in the toilet and they go into the bathroom.
I had an uncle that would leave poop in the toilet for a few days until he flushed it so that he could save money on the water bill.
However it was disgusting and I would never use his toilet.
The amount of water that it takes to flush the toilet is not that expensive.
As long as you're not in a severe drought then you should flush the toilet when you pee or poop.