Can I use word ‘ζωναρου’ in a Greek text for a female belt maker, or is zonarou idiomatic and maybe too demotic?

Ζωναρού would be the feminine of ζωναράς; that is the word for “belt-maker”, but it is far more common as a surname than as a profession. The feminine is grammatically correct, but you’re right, -ού feminines are now regarded as pejorative, because they are old-fashioned, and in olden times women either didn’t exercise professions, or exercised looked-down on professions—or else the suffix denoted a professional’s wife.

Thus

• μυλωνάς > μυλωνού “miller’s wife” (known from the proverb “from the miller’s wife’s arse, one expects no orthography”)
• καφετζής > καφετζού “café owner’s wife; fortune teller reading coffee cups”
• (modern, but unfortunately also pejorative) στριπτιζτζού “stripper” (as a peculiar mélange of English, Turkish, and Greek: striptease + Turkish –cı > Greek –dzis + Feminine Suffix –u).

All of them with negative connotations.

What’s a less stigmatised feminine? All of them would be awkward, but ζωνάρισσα is the least awkward to my ears.