It's just downright redundant at this point, no variety at all. I think the babies christening gowns have more uniqueness.
I kept wondering why I dislike this style on Catherine and you nailed it. The lapels do seem to be of a disproportionate size and there seems to be a gap or perhaps a missed hook or button at the waist. Overall I wasn't fussed on the ice blue colour as it seemed to make her look drained. But, that could be just because she looked so bright and joyful in the dress she wore for the christening.
I agree. Also consider the photos that will be taken. Kate knows from the previous christenings for her children, that in the official photos that baby will be seated on her lap and supported against her back. All that will really be visible are her hat, shoulders, arms and legs. Her torso will be blocked due to the baby being seated there. The focus of the photo will be on the child and the beautiful christening gown. The mother simply becomes part of the background IMHO.
All the more reason someone with an artistic background would have thought to wear something other then cream. Placing the star of a picture on a background they blend into, isn't visually appealing. She is meant to be background to him as you put it. Instead Louis and Kate blend into each other in similar shade. 
