They don’t have to be astronauts in the same way as as the programmer of a word processor does not have to work in office administration. But if said programmer has to write a letter and uses a different word processor you would raise an eye brow.
I think it definitely would be a good idea if the creators of a space suit wear it and try to do some light DIY with it.
I'm not sure. I can imagine that some programmer would prefer LaTeX, even if they work in the Google Docs team (or the Microsoft Office team). That wouldn't raise much of an eye brow from me: Google Docs isn't exactly aimed at programmers.
They don't need to be an astronaut to wear the prototypes and check how they fit, how they allow you to move or not move, how convenient it is to access the tool belt, etc.
The equivalent would be just wearing a helmet or a sleeve. If they are wearing the whole suite they are end to end testing in staging opposed to production.
I think it definitely would be a good idea if the creators of a space suit wear it and try to do some light DIY with it.