It's wholly appropriate in India.
Seeing that you're in India (from your name and because you use the term "non-veg" which is a peculiarly Indian idiom), let me assure you there is already huge local context and sensitivity to vegetarianism.
At most mature workplaces (organizations that have been in existence for more than 3 years, with more than 1000 personnel using the cafeteria) there will already be "vegetarian-only" microwaves. This is more common than you think, and is not a weird request in an Indian setting.
Someone in HR, if you have a HR department, is the right person to direct this request to. The actual implementation might be done by the Admin team if you have one. What I have seen is they keep some microwaves labelled "veg only". Of course this is on an honour system; no one will get punished if they reheat fish in that microwave, but generally people have enough sensitivity to how some vegetarians see the world.
I suggest before dropping an email, you first corner an HR person in the corridor informally, and ask her (note my gender bias) something on the lines of - "Hi _, do you have a moment? I was thinking of this for a while (or other filler sentence); is it possible to have one microwave in the cafeteria labelled as VEG ONLY? Some of my friends at company X said that it is a common practice, so I thought I'd check with you and see if we could do something similar?"
Don't mention the "smell of fish" etc. Such things might be perceived as offensive, like you're looking to rake up trouble, playing the Jain card or whatever. Best to keep it neutral.
This will give you sufficient feedback to how things are perceived at your workplace; do they have history with this topic, has it been tried or discussed before, etc.