How to switch off mDNSResponder as it is draining my mobile data?
mDNSResponder handles DNS resolution in macOS. That means that when any other program running on the Mac wants to connect to an Internet server by name (e.g. Safari wanting to connect to www.apple.com), mDNSResponder is what does the research to turn that name into a numeric IP address so the program can actually connect to it.

This means three things: First, if you turn it off, your Mac will be unable to do very much on the Internet. Everything that uses DNS names will break, and that's basically everything that uses the Internet.

Second, it shouldn't be using that much actual data; lookups are pretty small, and in normal usage it won't do too huge a number of them. It might look like a lot if you're using something that shows cumulative usage, since mDNSResponder runs basically forever, so you'd see how much data it's used since the last time you restarted your Mac.

Third, the data that it does use isn't really its fault (so to speak). It's doing things (and generating traffic) because other programs are doing things that require it.

If you are seeing a large amount of traffic from mDNSResponder, there may be something else going on. I'd start with a malware scan, and if that doesn't solve it look into finding out what all that traffic is and what program is triggering it, and stop it at that level.