I think that its fine to use your thumb for strumming. The more ways you can strum, the more versatile you will be in the long run. Thumb strumming will often give you a softer, more mellow tone than strumming with your other fingers, and usually less volume. You might find it to not be loud enough if you are playing in a group with other people. I would encourage you to keep experimenting with other fingers for strumming, the more options the better!
Using your index finger will give you brighter tone and often much more volume and will probably facilitate more strumming speed when needed. Keeping your finger really relaxed and letting your wrist do most of the work will keep it from getting "hung up" in the strings.
The most important thing is that what you are doing is fun and works for you! As long as you don't feel like its holding you back from being able to play what you want to play, then stick with it. Just remember that "variety is the spice of life"!