Why is there "/proc/net/snmp" file on my device? Could it be due to a malware?
There is no piece of software that has zero probability of being infected by some kind of malware. But in general, Windows by design is more prone to infections as compared to Linux and other UNIX based operating systems. Android is also based on Linux kernel, plus it has additional security mechanisms related to booting part, such as locked bootloader and Verified Boot. So generally we don't see Android devices hijacked by bootkits.

That said, init is the very first userspace process started by kernel with PID 1. It's a generic piece of AOSP, developed by Google and not modified by phone OEMs or network operaters (AFAIK). However its configuration files *.rc can be modified / added / removed easily to start / stop services and daemons on boot.

Presence of /proc/[PID]/net/snmp indicates that your kernel has ability to support an SNMP agent if run on your device. This is common, modern kernels are built so. Alarming is if there is an agent running in background on your device, something listening on UDP port 161 from an SNMP management console. You can analyze all listening ports using netstat or ss. Both are shipped with Android.