This is one example of jouyou-status not being everything: I find it's quite common to see this word written in kanji in many different types of texts, maybe about as commonly as I see it in kana.
Not to say you're not correct about the need for a usually kana marking, as other cases like this in the dictionary have it.
One thing to note is that even without the tag, ケンカ is already listed as an alternate form of this word, which also seems to be a common way these cases are presented in the dictionary.
From my own anecdotal experience, this is more commonly written in kanji. But of course totally depends on the kind of text you're reading.
喧嘩 Usually written with hiragana?
Since both kanji in 喧嘩 aren't in common use, I assume this common word would normally be written with hiragana? Should that be stated in the dictionary entry?