I'm an ISFJ and have always been great at spelling. I've won quite a few spelling bees in my time. However, my INFP husband is a writer and he is an awesome speller as well. We are both good with grammar too. I don't think it's a type thing per sae. I think it possibly has to do with hemispheres of our brain.
My youngest son is not good with spelling, grammar, etc (he's an ISFP), but he's excellent with math and science. I despise math and had to take a remedial math class in college for no credit just to take the basic Algebra class. You know how they say some people are right brained or left brained (I think they are balance brained people too)...I think we all are good with different things. I wish I was better at math!
I do think how much you read does relate to how well you can spell and your grammar. We didn't have a tv until I was 12 and I, so I read books like crazy. When I would take the annual testing at school, I would score off the charts on things like "figuring out the meaning of words by their context" and things like that. My sister, on the other hand (also an ISFJ, weirdly) was outside playing baseball or building treehouses with my brothers, rather than reading. She's terrible with spelling and grammar.
Anyway, I don't think it's related to personality type.