Unless she's perhaps not the weakest woman possible, just the weakest known. There may be levels (I'm not referring to the 21...) below Morgase where the ability isn't even really quantifiable by a channeler.

It's just that no Aes Sedai mentionning Morgase's strength did comment that it was unique or almost unique to find a woman so weak.

But yeah, it seems more likely women like Morgase are as rare as women around Lanfear's strength. Sorilea's strength is likely to be fairly rare too, if not as rare as Morgase's (perhaps more comparable to the rarity of women like Graendal).