For that case specifically, officially he only did a decapitation attack on the enemy flagship, and only 'just so happens' that the enemy fleet was set upon by crystal lifeforms shortly after. The fact that Hiro was the one whom called the lifeforms there to start with is not something anyone wants to advertise or have proof of anyways, since those singing crystals are highly illegal to have around.
Except for the fact that he was willing to take his single ship into an entire enemy fleet and take out the enemy flagship.
Exactly how is that not worth a medal? It's very contrived. In the real world, people have gotten higher honors and awards for doing much less than accidentally wiping out an enemy fleet.
The "well, he can't admit he had the crystal, so it's all swept under the rug" doesn't work because even with his bullshit explanation that Serena wasn't buying; he managed to defeat the fleet.
What, is the military going to go, "Wow. Yeah. You caused that invasion fleet to be destroyed... Buuuut you didn't do it solely on your own and it's because an alien crystal caused space creatures to destroy the fleet instead. So, thanks, but get fucked. It doesn't count."? All he'd have to do is point out he was the one willing to risk solo'ing an entire fleet and because of a lucky shot the fleet is now defeated. Not Hiro's fault that, supposedly, that the enemy fleet had such a dangerous thing on their ships and he hit it. Hell, that's even better! The enemy was obviously going to use this crystal on Serena's forces! He stopped that from happening!
In other words, medal up the son of bitch.