Is it bad to store a mac on top of a windows laptop due to the magnetic power cord?
Unless the laptop comes from the mid-1980s and has the floppy drive directly below the magnet, no.

The components most affected my magnetic fields are the hard disk drive and the CRT display. Laptops haven't had CRT displays for quite a while now, and are largely losing their magnetic storage. If you do have a spinning drive instead of a SSD, the current ones are far less sensitive to external magnetic fields than they used to be: increased capacity -> less sensitivity.

So unless you have a massively-powerful rare-earth or electromagnet on the shelf, don't worry about it. The tiny magnet in the power connector won't even be noticed.