I guess the variable is how much your recipient values presentation vs content, or in the case of ink and pens, where are their relative levels of Pen-y-ness vs Mail/Letter-y-ness. And then on the Letter side, how much interest in stationery vs erudition.
Off that chart would be "mail art" people where presentation is everything and the sentence or two that might be there is more typographical art. Or the Postcrossing people where I think the overwhelming point of interest is collecting sent-from locations, then maybe interesting cards, with what you write fairly irrelevant.
On the World Of Snail Mail forum I try to get a sense of how much, if at all, someone is into pens. If they don't care, I try not to do the sometimes annoying hobby thing (I see this with cameras and ham radio) of naming in detail the equipment used. Initially I'd assume minimal pen geekery. Someone from here, at least a modest level. My longest penpal from FPN started off "just into writing letters" but now updates me on latest pen news and I see more ink colors. Another convert <evil laugh>.
I don't think I've ever tried to use fancy stationery or interesting ink to make up for a boring letter, but I have put extra effort into it when a reply is really overdue. Plus maybe a photo tuck-in.
If it's a letter to a high level pen & paper geek and it's a fine nib, careless handwriting, with Quink Black on copy paper in a drugstore envelope then, yeah, wow, better write something fascinating, eh? Starting with, why _this_ particular black ink.