I think this would be a neat little feature to have. So players can have a good idea of the number of mobs they’ve had to kill to level up. Giving them a rough idea how they are going and what the requirements are like to gain a new level.
You can look in achievement (F4) in monster tab.
But that is troublesome though. It gives you the kills for each mob. Not total mobs you’ve killed in a map. Something that you can reset as well would be nice.
Can’t you just figure out the average % you gain per mob and then calculate how many you need for a level?
Personally, I feel like a mob counter would be a pointless feature. Usually when I am levelling I am more concerned about % gained over time (which is easy enough to calculate), rather than mobs per level.