Theory: the Shroobs are the reason why the tech of the Mushroom World is so haphazard.
See, the confusing thing about the Mushroom World is that while it's allegedly set mainly within a medieval-style kingdom where spears, heavy armor, steam trains, wooden ships, and dirt roads are the norm, the games (especially the RPGs) are chock-full of modern-day technologies like televisions, cars, neon signs, electric lights, jukeboxes, conveyor belts, bazookas, computers, pagers and cell phones (both with ringtones), rocket ships, jet planes, blimps, handheld video game consoles, and more.
While some of this could be chalked up to geniuses existing(E.Gadd, Bowser, Wario)I think there is also another explanation.
See, in Bowser's Inside Story we see Bowser has Shroobs frozen in his basement, very likely for research of their tech given how smart Bowser is. Given that Bowser did it, I think it is quite likely the other kingdoms of the world also snatched up some of the Shroobs to understand their advanced technology.
What we see in the Mushroom World today is the result: medieval kingdoms have basically leapfrogged ahead to modern levels of tech thanks to studying the Shroob, which is why the various Kingdoms are a weird mix of modern and medieval.
A side note is that this gets into sort of dark territory that the various kingdoms basically frozen an entire species and are experimenting on them to find their tech, but that's a note for another day.
TL;DR: the various Kingdoms studying Shroob Tech explains why the tech in the Mushroom World is both modern and medieval.