The View>Dock Icon command shows the various stats you can have the Dock Icon show, but AFAIK you can't have Activity Monitor display in the menu bar. There are probably numerous third party programs that can do that, though.
Also, under Window>Show Floating CPU Window...you can show that at move that window towards the top of the screen, but it won't be in the menu bar.
This program looks like what you might want. I use the widget version and it is very good.