A simple trick to get the Poison Mushroom in Super Mario 2, you can use this only in the Standard Ground theme, it is not available on others. It is pretty easy to get this item you need the Angry Sun to unlock Poison Mushroom.
How To Get Poison Mushroom
Switch to night mode by using the Angry Sun and in the item dial you can get the mushroom. Keep looking for the same and you will unlock few that you can place on your customized level. The same item was first introduced in Super Mario Bros: The Lost Level and now it appears back in Super Mario Maker 2.
Poison Mushroom is an item that can power down or kill the player if it is just touched by the enemy. So it is a quiet important item in the game and you can trick players in collecting it. You will need to check the purple item dial at the night time to get the mushrooms, so anyhow you have to turn the day to night in your level before accessing it.
You can now place the poison mushrooms in your levels and trick players collecting them out. Remember to turn your level to night mode and then check out the purple dial to gather the items, or else you will not find any.