No, I disagree. People will very likely want to install your mod again, whether it's a new version, after they've upgraded SMF, after they've fixed a bug in something else, or whatever.
Do NOT undo database changes when uninstalling a mod. Instead provide a page with instructions for how to do it manually, or even better, provide a separate fulluninstall.php file that they can run themselves to remove everything if they choose to. Mention these options in the uninstall readme.
For example, my pretty URLs mod will not remove the database changes. I actually recommend that if people choose to stop using my mod they install the reverter package so that their links will not break. This wouldn't be possible if I undid the database changes.