I have asked about your Zoom issue here. My feeling is that some features of Ubuntu such as zoom and night light are integrated into gnome-shell and not available for installation in other desktop environments such as Lubuntu's.
xzoom does address the basic issue of magnifying content (and is included in Ubuntu Mate), there is another program, kmag, which comes with more features. However, it should be noted that kmag is a qt-based application and so will pull in a lot of dependencies on Lubuntu 18.04. This should normally not be an issue with modern systems, but it is better to be aware. By the way, if you move up to the LXQt-based Lubuntu 18.10 or later, the additional dependencies will be minimal.
sudo apt install kmag
0 upgraded, 119 newly installed, 0 to remove and 0 not upgraded. Need
to get 21.9 MB of archives. After this operation, 115 MB of additional
disk space will be used.
sudo apt install --no-install-recommends kmag
0 upgraded, 99 newly installed, 0 to remove and 0 not upgraded. Need
to get 18.2 MB of archives. After this operation, 98.7 MB of
additional disk space will be used.
What you see on your system in terms of dependencies maybe less if you already have other qt-based applications installed.
Here's an animated gif of some of the features kmag provides.
The snap route
Kmag is available as a verified snap as well. Currently, the information provided by
snap info kmag doesn't indicate the actual disk space used in case that is a matter of concern. See my question How do I know what a snap will install? to know more.