@Crayv  
Those patterns are still used on stone.
 
Given that stone absorbs water less than wood and is therefore less likely to split when it freezes, it makes more sense as a material.
 
Covering the target completely in ice in the shortest amount of time would still take a great deal of skill. Like hitting specific parts of a dartboard instead of just the center.
 
Effectively, it’s a competition to see who can hit the right number of bulleyes the fastest, although there’s probably more strategy based on predicting how the ice will spread, along with trying to influence that.
 
Given how the targets are setup, I think that the archers need to get at least one arrow in each section, and maybe another in the center. However it works, random firing won’t do the trick. Getting two arrows in the same place would be a waste of a shot, so they can’t just aim for the same spot over and over.

@Sjogre  
Reason I think it’s wood is on the sides it has grain lines much like the wooden supports. The flecks on the target surface are in a ring like pattern. As for the color, maybe they dyed it for some odd reason.
 
It’s also odd that the archery is more about peppering a target so that it is covered rather than precise hits to the center (which I think further supports the fact that ponies are terrible shots).

@Crayv  
Stone is rarely perfectly smooth, and we don’t see any rings on there. The flecks used also look more like a stone pattern than any kind of grain.
 
And, of course, there’s the question of why they would make wood a bluish-gray, which is a stone kind of color.
 
Normal old-style archery targets are usually made of covered straw, rather than wood or stone.

@Sjogre  
Looks more like some type of wood as it seems to have a grain pattern to it. No clue how hard or soft that wood is.

@Crayv  
The targets look like they’re made of stone. Any penetration is impressive.

@Wlah  
I think having an arrow pass through your heart is gonna kill you regardless, in fact those arrows don’t seem to sink in that deep, so their piercing factor isn’t that strong… or maybe ponies are just really bad at using bows.

Cliffjumper (since his primary weapon since G1 was a “glass gas” launcher): “That’s my ponies!”

Imagine those arrows in actual combat.  
Hit the heart? freeze and die.  
Hit a wing? fall to your death, or shatter when you hit the ground.
 
Terrifying.

This is poetry at its finest.

Half the archers were disqualified for using performance-enhancing drugs.

The royal intelligence office dutifully keeps track of drug use in Equestria.