Watching Netflix even on mobile data does use a lot of data and can use up to 1 GB of mobile data per hour of watching Netflix.
You can go into your Netflix settings and change the data usage and video quality so you use less data when watching Netflix on your mobile device on the mobile cellular internet.
When watching Netflix it's best to connect to WiFi if possible to avoid using all of your Data too quickly although if you have an Unlimited Data plan then you can watch as much Netflix as you want too.
For High Definition Netflix movies and shows it can use up to 3 GB of data per hour unless you set the video quality to where it's not that good.