Something on top of a speaker=okay?

I don't know.  I might be concerned with the taxidermy chemicals being bad for the drivers.

And my posting asking if the air pressure in the listening room makes a difference got deleted?

No, something small and light won’t make and difference and neither will a little cat and dog hair on the grills.  

I keep mine symmetrical. One has a moon globe on it, the other a Mars globe that are both 12" in diameter. Seriously, I don't think anything on top of the speaker matters all that much as long as it doesn't overhang the front baffle or resonate at any frequency. YMMV. (I did put some rubber bumpers under those globes stands just for the heck of it). 

@ibmjunkman regarding air pressure, probably not at the slight natural range of pressure differences here on Earth. However, long ago on Skylab (1973-1974), where they kept the pressure at 5 psi of pure oxygen, the sound attenuated quickly, causing the astronauts to have to yell at others located at the other end of the thing where the Apollo Telescope Mount controls were located. I think studies were done on frequencies lost (mostly the highs) from the low pressure.

Today though, the ISS keeps its pressure identical to Earth’s sea level, so they don’t have to contend with that.

I would wonder if the relative humidity would make a difference and whether that difference was audible?