No side sleeping is not bad for your back and actually if you have back pain sleeping on your side may actually prevent back pain.
Sleeping on your side is the best position to sleep if you have bad back pain and I find it much more comfortable to sleep on my side than on my back.
I have back pain and it flares up and hurts more when I sleep on my back but when I sleep on my side I don't experience any back pain and sleep much better.
Some people sleep better on their back and some people sleep on their sides and some people sleep on their stomachs.
You should sleep in the position that is most comfortable for you.