I agree that yotsuba is top class in that regard, but I dunno about the mundane part. To me slice of life is a genre based on showing the everyday mundane life with an extraordinary twist in order to generate drama/humour. So it's not really aiming to be mundane as much as it is to make the mundane seem extraordinary, if that makes any sense.
Yotsuba is the same way. It's really just a story about a lone man taking care of a orphan who most of the times acts like a a little brat aka an actual 5 year old, but the extraordinary part comes from showing how her brattiness can effect other lives around her, and if the writing and characterisation didn't do such a great job in giving that brattiness so much charm, Yotsuba would not have worked so well as a premise.
Though slice of life is so broad that sometimes it's also about just showing the everyday life in an extraordinary setting. For example otoyomonogatari and Yokohama Kaidashi Kiko. Yokohama in particular takes an extraordinary setting and makes it mundane, but in doing so it just focuses and explores on the beauty of everyday life.