Only if you want people to remember it.

Pronounceable to who? A username doesn’t have to be English. It may be totally pronounceable in a different language and unless the username is in an audio medium it doesn’t ever need to be spoken so probably not.

No.

Na.

That seems unnecessary.

Realistically, no. But should as in recommended, yes.

If I read your username I’m going to read/say it in my head, so I’m going to attempt to pronounce it.

If every username was just a long number, I’d probably start to find a pronounceable, and specifically memorable substring to remember. A username of “846768556888434”, I’ll call you “34” from now on.

In the case of OP, “YICHM”, you’ll probably just be called “Y-CHIM” or maybe “Why Jim”. Is that your name? No. But is that what I’ll remember, probably.

But even pronounceable names, like mine, probably get shortened. Just “Mimic” or just “Jar”.

Which is all just to say I’m going to try and pronounce it no matter what you have for a username, so if you want others to pronounce it the same way, it should be pronounceable.