Why does Katniss put Seneca Krane's name on the dummy?

Answer 1

She didn't. She used did that to anger the game makers because when she stepped into the demo room, she realized the game makers were in turmoil and seemed frustrated to Peeta. She didn't want the game makers targeting Peeta so she do some outrageous demo by hanging a dummy of Seneca Crane to divert the wrath of the game makers onto her.

Answer 2

After the 74th Annual Hunger Games (The Hunger Games Katniss and Peeta were in in Book 1), Seneca Crane is killed because he panicked when Katniss and Peeta were going to eat the nightlock berries. He allowed both tributes to survive, contrary to what he was told to do. When Katniss and Peeta are reaped for the Quarter Quell (Book 2), she tries to come up with something to show the gamemakers on the final day of training. So she decides to hang a dummy in the middle of the training center. She splatters red paint (blood) on it and writes the name Seneca Crane.

Answer 3

To try and show them that the Capitol didn't own her. It was an act or rebellion.