"I am a little bit of an otaku."
I beg to differ. In certain places I've been to (that is not Japan) people flaunt that they are otakus or even sometimes being lolicons.
Depends on the situation, only if it's obvious you are talking about yourself. For Duolingo, it looks like they wanna translate everything directly.
I think it might sound better to say 私はちょっとしたオタクです. It seems オタク is usually used as a 名詞 rather than a 形容動詞.
> doesn't accept 私... wants the hiragana...
> some previous lessons fail if you use the hiragana rather than the kanji
pls duolingo. pls.