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.