Often the hover hints are in kanji when there are only kana to use which sort of makes them pointless.
I think the point is to get you to learn the "readings" of the Kanji, although I agree that it is difficult. Kanji often have multiple "on-yomi" and "kun-yomi" readings depending on the context they're used in. This is one of the most difficult parts of learning Japanese and why many people become proficient in spoken Japanese long before they are literate (which doesn't lend itself as well to the DL system designed for European languages).
A common solution in written Japanese for children and foreign learners is "furigana", little Kana above the Kanji to show the reading. These are also used for Kanji beyond the Jouyou (set taught in schools) when in print media such as Newspapers and tourist brochures. I think there are applets that will mark digital Kanji for you to read Japanese online. These are very small though so I imagine Duolingo is struggling with making these large enough to read, especially on mobile platforms.
I suggest you also supplement Japanese learning with a kanji study app, I find this one really helpful.
I'll confess that I will, on occasion, resort to copying kanji into google translate to figure out how to type them. It seems to function as a workaround, although I hope they manage to implement something better in time.
I just wanted to lodge a complaint about the tiny kanji in the hoverhints. What are they thinking? I use a desktop mac and I cannot see them clearly even with my reading glasses on. Plus I don't know them. They should use hiragana either in addition or alone. Sometimes, they even have TWO really complicated kanji as hover hints. Grrr.