Anwar, as to that remark I disagree. Yes he was perhaps being intolerant. But most uniformed organizations have really strict regulations about jewellery, without distinctions between what's mere fashion and what can be deemed "cultural".
It's not like she'd be forbidden from wearing it when she's off-duty; and it sets up the final scene nicely where she accepts joining on condition she be allowed to wear the earring, and Picard agrees to it.
Any regulation starfleet has toward uniforms is lax at best. Riker was just using that regulation as an excuse to act like a jerk towards Ro a fact which becomes quite clear a couple scenes later when they are assembled in the magic meeting room and lo and behold Worf has his klingon sash and Troi isnt even in uniform. Yet no one tells them off for breaking the very lax startfleet uniform regulations.