I think it would be beneficial if it was clearly shown which words are conversational and which words are used in writing. I’d even go as far to say formality should also be shown. Even if there was just a symbol on the “card” itself that indicated written vs casual, or formal vs informal.
For example, 昨年 and 去年 both mean “last year” but the former is used in writing and is more formal whilst the latter is used in day-to-day conversation. Quite frequently, I use words that I learned on Wani Kani only to find out that they aren’t used in casual speech, but rather written speech.