It's not possible to change the font at this time. Since the notification is artwork, and not a rendered display, having different fonts would require a different set of numbers built into the app for each font. This would increase the size of the app.
So, for now we try and provide an option that works best across all the devices though this does cause the number to stretch/squeeze on some displays with the huge variety of options in the market. We haven't ruled out additional fonts for a future update, but don't have any definitive plans.