I don't mind costumes - animal or otherwise - providing they don't detract or interfere with the dancing. Natalie's dress was fine; she should have just kept the penguin "waddling" to the first and last couple of bars of music, and choreographed a proper cha cha in between. The music didn't help, but then that is frequently the case in Strictly.
Kirsty's outfit was ridiculous. She was doing (if I remember correctly?) a foxtrot, the footwork and leg placements for the lady is really difficult, and more often or not, beginners have a tendency to look as if they are walking backwards with their legs apart instead of together. A long dress covers a multitude of sins, while Kirsty's costume made them all too obvious.
And remember that dreadful lobster costume one of the celebs had last year? I doubt the best dancer in the world could have made that look good.