Why do you think that? The statement about the DNA usage is given only during the battle against Lavos, when the party members first see Lavos' second form (and apparently deduce the fact just by looking at the creature). The DNA is probably passed on to the spawns for evolution purpose, but I don't think it's absolutely necessary to their birth per se. Well, or maybe it is, but it seems it has to affect the parent Lavos first, before the spawning.
Well, I guess I have my answer now... The Lavos Core looks alien because he also possesses the DNA of the planet he was born on. (Could it have been a planet of humanoid ducks with Scottish Terrier heads?)