I have a Moment 25-mm M-series macro lens. It works great in situations where you control light and movement, for example, photographing plants at home under artificial light. I have some sample photos online.
I tried using it in the field but the quality of the results do not outweigh the inconvenience, even with the excellent Moment phone case. Without control of light and movement, macro photography with an iPhone is a crapshoot. I’m not sure if that’s true of all smartphones but I’d be surprised if it isn’t.