Ok, so it seems implied that actually Momo's df is not a failure but true to the original. Vegapunk could have lied in order to not give such a weapon to the corrupted/evil WG. He could be the kind of scientist who loves his work but could also see what the rulers could do with his creations (let's say this is like giving a nuke to Hitler).
So he said he was a failure and Momo eventually get it so now, and more importantly in the future, we will have a dragon as a ruler and protector of the Wano country.