I’m assuming that you were referring to the idea that the testimony of two women is equal to that of one male. That idea most likely comes from Qur-an 2.282, the Qur-an’s longest verse, which deals with what we might call “contract law”. It’s really too long to quote in its entirety, so I’ll tell you that it begins
“O you who have believed, whenever you contract a debt from one another for a known term, commit it two writing”
A translation might go on to eventually say
“and call upon two of your men as witnesses; but if two men are not there, then let there be one man and two women as witnesses from among those acceptable to you so that if one of the two women should errs, the other might remind her.”
Obviously, without referring to all the rulings surrounding this and their evidences, when a woman is called to witness a contract involving debt, or to testify about it, she should have another woman with her, whereas this is not required for a man. There is nothing in the language to imply “mental fitness”. If one says the implication is that women are more likely to be err in testimonies about debts and contracts, that is also incorrect. The wording in Arabic is strictly “if” as in “if she errs”, not “when she errs” or “because she will err”. The text is not saying that a woman will err, it is only saying that if she does, another woman should be there to remind her. Either way, there is neither a linguistic nor logical basis to infer that women are seen as less “mentally fit” (intelligent, capable, intellectually mature, reliable) than men.