Is she sinful or blameworthy for her mother's death after consuming herbs that her mother warned her not to cook, causing her death?

If the reality is as you mentioned, then you are sinful and disobedient to your mother, and you bear a sin because you know that she is harmed by your advice and prohibitions. This act of yours is considered semi-intentional killing, and you are liable for blood money (diyya) and expiation (kaffara): freeing a believing slave. If you cannot find one, then fast for two consecutive months, in addition to repenting to Allah.