Not necessarily nasty, but unpopular. I agree with you. Some nursery rhymes have have been decrypted but many still lie in obscurity. Given that the perpetrators at the time were likely at the very least to have their tongues torn out and hands amputated, or at the worst suffer the extreme penalty at Tyburn, it is no wonder that they covered their tracks well. People of the time, though, could not have missed the significance. Teaching innocent children to chant political slogans continued till the 20C. They were untouchable.