You asked

Is the yongnuo yn25mm f 1 7m good in low light?

Here is what the site says about that.

From Yongnuo YN25mm f/1.7M: Yongnuo YN25mm f/1.7M: A prime lens by Yongnuo for Micro Four Thirds cameras, 25mm f/1.7. Specs - Maker: Yongnuo - Focal length: 25mm (prime) - Maximum aperture: f/1.7 - Made for: Micro Four Thirds sensors - Mounts: Micro Four Thirds - Stabilised: no - Autofocus: yes What it frames like - On Micro Four Thirds (Micro Four Thirds): like 50mm on full frame; f/1.7 gives about the depth of field of f/3.4 there. ...

From Sensor sizes: Is full frame worth it?: For low light, events, and portraits with very shallow depth of field, yes. For travel, street, wildlife on a budget, and most hobby photography, a good APS-C camera with good lenses is enough, and you'll carry it more. Lenses and light matter more than the last stop of sensor performance.

Quoted from this site's pages.

From these pages