It sounds odd unless for emphasis. I think it would be safe to avoid saying "mimi nina___" in general usage. Also people often drop the "ni" for "napanda" as well as it's implied when you are the one speaking.
I've definitely heard it in other dialects too, but I can't say which ones. It's a pretty logical development of language and has parallels in other languages, not only Swahili but also French, for example, with "moi, je ..."