Different strumming techniques make different sounds. When I was first beginning on the uke, I used my thumb to strum while I was still figuring out how to make the chords. I soon switched to my index finger (nail pointed down) and found this a little easier to play while making a bigger sound. I still use my thumb occasionally when I want a mellower sound.
There are many other strumming techniques, and they all sound different. As you progress on the uke, try working on some other strums and see what sounds you can make. The song I am working on now uses four different strums over the course of the song.