I suggest that you read a number of peer-reviewed research papers, preferably from Google Scholar or similar websites, about mobile banking in general and about its impact on low-income economies in particular. Afterwards, you can move on selecting the proper approach.
My suggestion is economic development as mobile banking requires infrastructure development, ICT literacy, mobile penetration etc. Your choice will depend on the data as well. If you take secondary data like financial statements of banks, the annual frequency would not make sense to look at them from a development perspective.
May take either e-development or e-growth. I believe it is a bigger topic especially from an African and low-income perspective. I recommend you take this as financial services inclusion and economic development. For further guidance, you may see the paper of Beck et al. (2009) Financial institutions and markets across countries and over time-data and analysis.
A small-scale study of this topic can be a research paper and a bigger scale can be a thesis topic.
Being able to pick up on this economic growth is a good topic especially in low-income Africa. my advice consider this as financial services inclusion and economic development.