Is it bad form to cold-email professors to do post-baccalaureate research? Suppose I am interested in doing more research in a particular area in order to strengthen PhD applications, but I have just finished my undergrad degree.
Is it considered bad form to cold-email professors who say on their websites that they are accepting undergrad researchers in their lab asking to do research this summer/next year, though I’m no longer an undergrad?
If you are not an undergrad, you cannot be hired as an undergrad. Most undergrads are unpaid (don't work for free!), and of those that are paid, many are paid using funds that can only go to enrolled students (the percentage varies by field). Besides, most school-year undergrad positions are on the order of 10 hours/week, although some summer positions can be full-time. 

According to @J.., sometimes new grads can be hired for full-time summer internships in a transitory capacity. Usually, this would be people continuing to work in labs they have already started with, or professors hiring someone they're familiar with. I find it unlikely a professor would hire someone new for only a few months, and if the opportunities were available, they'd likely to be posted.

Now that you have your degree, apply for jobs. 

Cold emailing isn't a good strategy for that (especially not in the current climate), mostly because budgets usually don't allow to hire as many people as you need, much less to have money laying around waiting for someone to ask for it. Apply for posted jobs and work your network.