I have all my PCs on top of my desk for a reason. While you can put a piece of wood on the carpet to elevate the PC a little, it's what you don't see that is a problem: dust being kicked up by people and even pets walking on your carpet next to the PC.
If you've ever had sunlight shine directly onto your sofa and you hit it, you'll see dust particles go flying in the air no matter how clean you are. Carpet is 10x worse in particle accumulation, and all that is being sucked into your PC unless you have good air filters.
Office carpet is not like home carpet. Office carpet is very thin more like an outdoor mat and does not trap dust and other things like deeper home carpet does (reason for that too: ease of maintenance). But if you are that short on desk space, put it on a flat board like an unused shelf board to get it off the carpet. I had one holdout PC that was set up just like that, and once I put it on the desk, the dust accumulation rate in the filters was cut by more than half.