I have a 486 (also another 286 mainboard) with a Hercules card, the plug is the same as RS-232. Is it possible to connect the output with serial port USB adapter and simulate Hercules-compatible monochrome display on a modern PC?
Unfortunately, Hercules plug, while being the same DE-9 plug/socket as "small" RS232, is completely electrically incompatible - other than using the same connector type, there is absolutely nothing in common between the two interfaces. You can damage the card, monitor or RS232 port/converter if you try connecting RS232 with the Hercules port.
So, no, neither USB-to-RS232 converter, nor genuine RS232 is going to help you here.
I don't know what you can do to create a display for your 486 (other than getting an ISA VGA card in place of your Hercules), but RS232 is definitely not the way to go about it.
If you mean the original Hercules graphics card (picture further up), it had a video port and a parallel printer port.
Neither can be connected to a RS-232 serial port in a sensible way. And even if you could, there's no way you can simulate a graphics card over an RS-232 connection: The graphics card can be only controlled via the ISA bus.
If you want to simulate a Hercules card on a modern PC, you can do this with emulator software like dosbox.
The Hercules / MDA plug is indeed the same physical as the RS-232 serial port (although 25 pin serial was also used) but the signals are very different. It's absurdly expensive to use this signal for anything these days as the only usage left is industrial but converters and even LCD monitors exist.
At one time, MDA to VGA/DVI boxes existed and I found a product called usb2isa which allowed plugging ISA cards into modern PCs via the USB bus but these seem to be not available any more.
A serial port (let's say 38,400 bits per second), besides being electrically incompatible with Hercules, is far too slow to handle the video bandwidth (720 bits x 348 bits x 50 frames per second = 12,528,000 bits per second).
Hercules is essentially an MDA-compatible monochrome card, and a GBS-8219 converts MDA to 800x600 VGA, so with that all you need is a USB video capture card that supports VGA input. But I think it will be cheaper to connect the GBS-8219 directly to a VGA monitor.
Or a Raspberry Pi supports up to about 14-20 MHz on its GPIO pins, so maybe you can write something to read the Hercules signal and display it on screen, then connect to it from your PC with VNC. Which, by the way, you could do through a USB serial adapter by running a SLIP server on your PC, but that would be silly.