It's only 'bad' if you don't like frogs.

So this is after beating Magus but before beating the Tyrano's Lair, right?

Actually, no. You're thinking of the Reptite ending, where the Reptites triumphed over the Ancient Humans and everyone in the future become reptilian instead of mammalian.
Here, this is what happens after you beat Yakra in 600 AD but face Lavos right after returning to the present. The king in the past falls ill, and because you didn't go back and help protect the kingdom, he dies. The queen then marries Frog and the royal line becomes descendants of Frog, thus Marle become part frog.

She looks gorgeous :3

Hey look thats me who got the thing!
Still amazing, thank you <3