The player used a Fireball, a power with the Fire keyword in a room full of flammable objects, catching anything that happened to be in his burst 3. That's how it goes. Just like every edition before it.
So? And the bard mocked a skeletton.
You have argued, together with others, that the name "Mockery" does not necessarily mean that mocks are involved.
The same way the player can argue that just because the spell is called fireball it is not a ball of fire and that according to the rules (the important things which stay fixed and can't be changed on a whim) only creatures are affected.
You seem to think that the creativity in 4E comes from the player now trying to find a spell description which lets him deal fire damage to the enemies but not ignite the room they are in. But for me this is on the same levels as players trying to find "Bag of Rats" loopholes (It is a form of creativity, but nothing I want to encourage).
For me, and likely for many people who think 4E lacks realism and left because of it, creativity means that the player now has to think of a way to attack the enemies without damaging the room they are in with the tools he has, and not to simply reshape the tools to do what he wants.
Also, by using "flavor is mallable" players can never be sure what their characters can do and what not because they only have the rules part of their powers and the constant danger of DM fiat.
Sure, you can be a "good DM" (as defined by 4E) and say yes to everything, but for me (as player) this is boring as it removes the need to be really creative as I can simply change the flavor to whatever I need and yet also frustrating because in the cases it doesn't work (DM says no) the only reason for it is DM fiat and not because I was not skilled/clever enough.