Hate DD’s planned baby name. Is it ever ok to say that?

[Giggsie]

I expect the answer is to keep quiet but I fear by future grandchild will be teased mercilessly.

I quite like the name. I'm not sure they'd be teased. Children accept names generally and there are many, many more unusual ones out there.
Would your daughter consider Ino? (A sea goddess in the Odyssey, though can't confirm she necessarily didn't have a tragic backstory - just about all women of myth did).
I have met an adult Io once; I believe they are Greek. Unusual and could be difficult for people to grasp the pronunciation but once that's grasped it'll be fine.

Sorry OP but I love it. You might suggest Iona as an alternative.

Could you say ‘Oh Io is so cute! Would you consider Ione/Iona for the birth certificate, just to give her an option for later on?’

Ione (Eye oh nee) is also Greek and means violet ❤️

@Giggsie
Jupiter's moon Io was named after the Io of Greek mythology too.

It makes me think of that Christmas carol - oh io io io let steeple bells be swungen. It wouldn’t be my choice but not your kid so I think you just have to keep quiet.

The baby is six months old now!

I pointed that out a few days again.
I wonder if they went for the name.

It’s not up to you. Io is all right anyway - although I prefer Iona.