This is a minor thing, but Kubuntu doesn't use the best font anti-aliasing out of the box and always seems to require some tweaking. This periodically comes up online, e.g. https://www.reddit.com/r/kde/comments/7ae1cf/kde_papercuts/dp9fzte/?utm_content=permalink&utm_medium=front&utm_source=reddit&utm_name=kde
In System Settings > Fonts, Kubuntu's standard setting for "Use anti-aliasing" is "System Settings".
I recommend changing this to "Enabled", and in the "Configure..." dialog, set "Sub-pixel rendering type" to "RGB" and "Hinting style" to "Slight"
Across many machines I've done this for--laptops and desktops alike--these always seem to be the best settings.