Whether or not you can wear jewelry in Granada, Nicaragua and feel safe I guess depends on the type of jewelry you wear. I wear a watch, my wedding band, earrings and necklace each and every day. They are not flashy so it does not draw much attention.
I would not recommend you wear your most expensive pieces, heirlooms or something that will bring your world down if it would get stolen. In all honesty, your iPhone is more likely to get stolen than your jewelry. Also, take taxis if out late at night or in areas that are not highly populated. If you can afford that 2 karat diamond ring, you can afford a 50 cent cab ride!