I think the fact that Shrek is an ogre, and the fact that Fiona's cursed form is also an ogre, is just a coincidence. The last part is key: "Then take love's true form."
It's not the "true form" of the abstract concept of "love". "Love" here refers to the other person in "true love's first kiss". Whoever her true love is, she will take their form upon their first kiss.
For example, she's a human by day and an ogre by night. We know that. If her true love were Donkey, then after the first kiss, she would have turned into a donkey. So back to the thesis: her cursed form is an ogre. Shrek is also an ogre. I contend that this is only a coincidence. The reason she turns into an ogre after the kiss is not because of the ogre part of the original curse, but rather because Shrek is an ogre.
Her mother is, apparently, human. Her father was a frog who turned into a human. Assuming the "turning into" affects miosis (not a typo, thank you anyway Firefox, but I'm talking about the production of gametes)*, the two should have a 100% human baby, Fiona.
