I know you can bypass all insets in the mixer view but let’s say you want to a/b with your eyes closed to hear the difference.using a key of the keyboard and not the mouse.
Yes, you can create a keyboard shortcut that will toggle. Assign a key to bypass Inserts. This is under File>Key Commands. Once you have a key assigned all that you have to do is ensure is that the mixconsole window is the active window because the the bypass Insert function is part of the mixconsole functionality. If you don’t have the mixconsole selected then the keyboard shortcut will not work.
the op just wants the selected one plugin to bypass.
that’s not key-commandable i’m afraid. i guess you could use the a/b function whilst disengaging all processing on the plugin for the ‘b’, but that is not a one-keypress type of operation i’m afraid. (the setting up, that is; not the toggling part.)
having said that, in generic remote, you will find commands to bypass an insert (#1 - #8) on the selected channel. that could be a way.
I found for a lot of things that switching to a trackball made it easy to set the pointer on the button and it says there, then just use the button with eyes closed. much better than a mouse.
The trackball idea sounds interesting. A similar idea, we can purchase a cheap travel laser mouse and plug the laser hole by a sheet of paper to not move the cursor. On PC you can hook up several mice to a PC (don’t know about Mac) and use it as a second mouse.
I’ve been suffered from wrist pain for a long time, especially with the mouse wheel. Thought I could use the left hand for scroll. Sounds like a nice idea, no? 
The second mouse for scroll only has been pretty handy to me. I can even accelerate the scroll speed by using two wheels on my both hands 
Of course, this also makes it possible to click the a/b button without moving the cursor position.
The Generic Remote functionality can do this.
Found that Entry while searching for the same Problem.
Now i have Cubase 8.5 and my hope was in vain, still missing that “Feature” 
your only safe bet as of now is going the generic remote route. it is a bit of a hassle to set up indeed, but it is one area worth exploring (as well as an area worth making more user friendly by steinberg). i’d advice to save and backup your generic remote settings once you’re up and running because when your cubase / preferences get corrupted (and they will), you may end up parting with all your GR assignments.
Can you tell us how to achieve this? I am new to Cubase and the quick controls / generic remote can be very confusing.