Under the penalty section of the config.yml there is a section labeled mob_rob_from_player. If set to 10 it will remove 10 currency from the player, if set to 10% it will NOT remove 10% of the player's currency.

config.yml
' penalty:

# If a player flies at any point in a fight, this penalty will be applied
flyingPenalty: 0.2

# This is the penalty if the player gets killed by a mob.
# Set mob_rob_from_player=10 to let the mob steal 10 dollars
#  or 10% to let the mob steal 10% of the players balance.
# Set mob_rob_from_player=0 to disable this
mob_rob_from_player: 10%'

Console Output with debug on
[12:41:37 INFO]: [MobHunting][Debug] Smashedbotatos died - Killstreak ended [12:41:37 INFO]: [MobHunting][Debug] Smashedbotatos was killed by a Husk [12:41:37 INFO]: [MobHunting][Debug] There is NO penalty for being killed by a Husk [12:41:37 INFO]: [D] Smashedbotatos was slain by Husk

You have written

10%'

Remove the ' and it will work. Im am testing if it ends with %. But be cause you have written ' then it does not work

After changing the configuration to the following and restarting the server.
mob_rob_from_player: 15%

I am still seeing the following result.
[12:56:36 INFO]: [MobHunting][Debug] Smashedbotatos was killed by a Wolf [12:56:36 INFO]: [MobHunting][Debug] There is NO penalty for being killed by a Wolf [12:56:36 INFO]: [Death] Smashedbotatos was slain by Wolf

Yes, this looks wrong. I will have check this.