Can I just grab a bag of patching cement from a hardware store?

For a small cosmetic touch-up, yes. For anything larger, a bagged general-purpose patching cement is not the same as an exterior parging mix. Exterior parging mixes for this climate are polymer-modified and fibre-reinforced, which helps them grip the wall and resist cracking through repeated freezing and thawing. General patching products are made for small repairs and tend to shrink, crack or lose their bond when spread as a coat over a whole wall. If you do buy a bagged product, check that it is rated for exterior foundation parging, follow the temperature and curing instructions on the bag, and prepare the wall properly first. Our guide to what parging is explains the main mix types and when each one makes sense.