It’s rude in two ways.
For one: any vintage typewriter makes enough staccato noise to be heard despite headphones (quite apart from the fact that the statement ‘everyone has airpods on’ is obviously no truer than when a child whines ‘Everyone in my class does it!’), so this is a deliberate noise nuisance, whatever she may say.
For two: it’s not a customer’s business to take it on herself to “add to the ambiance” of any establishment, and her blithe presumption that both the management and all the other customers are and ought to be thrilled to have her clacking away so steampunk-ly is breathtakingly conceited. Being conceited is just a character flaw; but inflicting one’s conceit on the general public is rude.
A rough parallel; if I go and sit in a coffee shop wearing late Elizabethan gear, starched ruff, wheel farthingale and all, because I’ve just done two hours’ historical presentation to a class of 9-year-olds and I need caffeine before going back to cope with the 10-year-olds, fine. But if I dress like that just to go to the coffee shop because I feel good in it and I think everyone will be so thrilled to see me there that they won’t mind a bit that my outfit is blocking two aisles, not so fine.