Is hip hop or rap music with unlawful lyrics alright as long as it is not accompanied by musical instruments?

Music, if accompanied by musical instruments, is haram, whether what you’re singing about is halal or not. That which is not accompanied by musical instruments becomes haram if the content is haram.

"And of mankind is he who purchases idle talks (i.e. music, singing, etc.) to mislead (men) from the Path of Allah without knowledge, and takes it (the Path of Allah, the Verses of the Qur'an) by way of mockery.  For such there will be a humiliating torment (in the Hell-Fire).” [1]

Narrated Abu 'Amir or Abu Malik Al-Ash'ari that he heard the Prophet (ﷺ) saying, "From among my followers there will be some people who will consider illegal sexual intercourse, the wearing of silk, the drinking of alcoholic drinks, and the use of musical instruments [ma'aazif] as lawful. And (from them), there will be some who will stay near the side of a mountain, and in the evening their shepherd will come to them with their sheep and ask them for something, but they will say to him, 'Return to us tomorrow.' Allah will destroy them during the night and will let the mountain fall on them, and Allah will transform the rest of them into monkeys and pigs and they will remain so till the Day of Resurrection." [2]

Note: Here Rasulullah (ﷺ) has mentioned music with drinking of alcohol and fornication (zina), which are of the MAJOR (kabair) sins!

References:

[1] Surah Luqman, 31:6

[2] Sahih al-Bukhari, Volume 7, Book 69, Hadith: 494