TBH I always thought MUI was a developer tool for people that couldn't actually code, but could wave a mouse and click occasionally. To the extent that if an app needed MUI, I just didn't bother with that tool. Quite a Luddite attitude to some, but... I guess I wasn't interested in a flash GUI that spent all it's time and resources looking pretty.
Heh, not at all. MUI was (and still is) an advanced and powerful GUI toolkit which just offers much more than other toolkits did at that time. Although it was relatively easy for programmers, it doesn't mean it was for those who can't code, and there wasn't mouse involved with the coding... I don't know where did you get that

Beauty of MUI is that programmers don't have to make eyecandy and flashy or themeable GUI:s themselves, they just code the programs, but then users can configure and make whatever they want to the look of a program. Users can configure the looks of all MUI programs by global prefs, but they also can override them for each single program or even window. It's quite win-win situation, because coders can go with less work, but users have more options to tune the look/functionality of programs to their liking.
The original MUI isn't that heavy with basic settings. Of course it doesn't run well on 68000, but 020 or above isn't problem. MUI became quite a de facto standard on later days and you wouldn't have practically any Internet software without it, or pretty lonely with them at least

It was a good thing for higher end Amigas and later days/future.