Is writing to prospective PIs during admissions season a good idea?
It's usually better to contact a potential PI than to not contact them... but you're timing is a little awkward.  The thing about contacting them is that you want to be pointed in what you're communicating.  Although asking if they are accepting students is relevant, it seems a little late for that because you've already applied.  

In other words, you can't just send the typical email because it doesn't seem as relevant, even so here a couple of ways of reworking the typical approach:


Briefly discuss your interests and why you're a good fit for the lab. Then simply state that you want to alert them to your application. ...If they are interested, they will look up your application.
The other thing that comes to mind is for if you are offered an interview, which should be happening around now. You can email the professors you are interested in at that point in time and briefly discuss your interests and why you're a good fit for the lab, then say that you're excited about interviews and you hope you have a chance to meet with them at that time.


Yes, professors are busy, and you may not get a response, but I don't think writing an email could lead someone to throw out your application which they otherwise would have considered. In other words, if your email is polite and nothing weird, then I don't see how it could have a negative impact.

Hope this helps and best of luck.