The basic difference is that: Mum weighs herself: Happens regularly but not necessarily at this moment. Mum is weighing herself: Happening now. She is on the scales There could be confusion if perhaps she is on a diet and she is doing it a lot over a short period of time. You might say, she is weighing herself every day for the next two weeks. We use the continuous here to emphasize that it is temporary.

since its an act of weighing, happening at the time, it can/should be present progressive tense. you can say ''my mom WEIGHS 55 kilos' then it would be correct. But mom is doing the action so it should be progressive :)