OK, so Fiona’s curse goes:
By night one way,
By day another,
Until you find true love’s kiss,
And then take love’s true form.
True love. True love. So why didn't Queen Lillian kiss Fiona? As Frozen and Maleficent pointed out, true love doesn't have to be romantic, it can be between family or even friends. Also, King Harold seemed to be more prejudiced against his daughter’s curse and anyway, he was born a frog, whereas Queen Lillian was born human and loved Fiona enough that she accepted Shrek into the family. So why didn't she kiss Fiona?