Mimblewimble is about simplicity and elegance which benefit both scalability and privacy. It solves 2 of the 3 major privacy issues (amounts and addresses), while leaving potential to reduce the 3rd (linkability) by making coinjoins trivial (tx aggregation). Monero has much better privacy, but it pays a heavy price in having 20x more chain bloat, and needing to have random access to some info of all spent outputs. It also lacks (scriptless) scripts which makes possible things like payment channels.