Clicking on Activity Monitor's Dock icon won't maximize the window
I've confirmed this behavior as well on 10.10.4 on a MacBook Air.  Drove me nuts until I came across this post confirming others are experiencing the same behavior.

Fortunately there is a workaround.

If Activity Monitor has been minimized to the Dock and you need to un-minimize it, click its Dock icon to bring the app to the foreground.  Then, to actually reveal the Activity Monitor window, press Command-1.  Command-1 is a keyboard shortcut listed in AM's "Window" menu purposed to give focus to the "Activity Monitor" window.

Alternatively, instead of minimizing AM to the Dock, you could hide it with Command-h.  Unhiding by clicking AM's icon in the Dock doesn't appear to be an issue.