Sometimes your mouse is a zoom tool, sometimes it isn’t. Sometimes you have easy acess to your navigator panel, sometimes you don’t. You can always, however, use “Ctl/CmdCtl/Cmd +” and “Ctl/Cmd -” for zooming in and out.
These work in every view (Grid, Loupe, Compare, Survey) in the Library module and in every mode in the Develop module — even when you are using tools such as the adjustment brush or spot removal tool. They work in Camera Raw and Photoshop as well. (True, they don’t work in the output modules in Lightroom, but zooming is not available at all there.)
The first time you apply “Ctl/Cmd +”, the image goes from Fit to Fill, then to 1:1, then to the last zoom ratio you have set in your navigator panel (e.g. 2:1). “Ctl/Cmd -” zooms out in the same steps.
Finally, when you are zoomed in, if you hold down the space bar, the cursor becomes the hand tool, so you can click and drag to move around in your image. Again, this works everywhere in Lightroom and also in Photoshop.