Smart View is like display your phone on smart tv or smart view supported displays which is supported by most of the galaxy device
But in Dex Mode you can operate on phone big screen to do some work or etc which give you desktop like experience which very different than smart view it has its own UI and you can use your device as keyboard and Mouse. Note that it is only available for flagship phones of samsung.
smart view just to mirror your mobile screen while on dex you can use to perform so many task For more info on dex you can watch this video attached with this link
