The Balcon is flat and right in the centre. Lots of bars, restaurants etc and a great place to people watch. The nearest beach to the Balcon is down quite a steep slope though and it doesn't sound as if your Mum would manage it. The roads leading off the Balcon where many of the shops are, are hills.
The Monica may be a better bet as it may be easier for her to get onto a beach. The beaches are mainly sandy and most of the Nerja beaches have some sunbeds for hire, but I'm not very familiar with Torrecillo, which is the nearest to the Monica. There are also bars and restaurants within reach of the Monica, and some souvenier shops. It would probably be too far for her to walk from there to the Balcon.
For proximity for someone with walking difficulties, the Monica would probably be better than the Perla Marina which is just slightly further out.
Does your Mum intend to stay around the hotel area? What is she looking for from her holiday?
I'm wondering if Torre del Mar might be a better bet for her as it is flatter. It's not such a pretty town as Nerja but it is more Spanish. There are lots of bars and restaurants. In the evenings there is a wonderful atmosphere along the paseo with many Spanish families out for a stroll and during the summer there is an evening market on the beach. Prices are also much cheaper than in Nerja
I live pretty much midway between the two areas and Torre would be my first choice but then I'm not a tourist and don't know what your Mum's idea of a good holiday would be.