When someone says someone has a half baked idea it means that the persons idea is not well thought out and the idea they have is likely to be a bad one.
The idiom half baked idea is an idea someone has that is not well planned or carefully considered, ill-conceived, unsound or badly thought-out.
In other words if your idea is half baked it may work and may be a good idea but it's unlikely that the half baked idea you have is gonna turn out like you thought or hoped it would.
So it's best to have a well baked idea which means that your idea is likely to succeed and you thought the well baked idea through.