Why did iZombie add gourmet cooking shots in the second season?

It puts an emphasis on the brain's importance but also how sometimes they are just left lying around and treated as normal food. This puts us the audience on edge whenever we see food that is Liv's so we realise something is happening or about to happen. Her eating Lowe's leftovers leads to her figuring out Blaine is murdering, it is also the reason Glida breaks back in after turning.

I think it is used to emphasize the method of cooking. I have noticed in season 4, that each meal reflects the personnality of the victim. Episode one, the victim is a football fan, Liv makes "chicken wings". Episode two, the victim is a high class older lady that enjoys her drinks, Liv makes a cocktail. Episode three and four, the victim is a hopeless romantic that enjoys cooking, Liv makes heart shaped chocolates. Episode five, the victim is a canadian hockey player, Liv makes a poutine. I hadn't noticed the corrolation in the prior seasons. But this adds to the overall effect of the setup.

Also be noted that in the first season, Ravi wasn't aware of Liv's condition until further along in the season, meaning she would have to hide to consume her meals, whereas as the seasons progress, there is less need for her to hide and she can take time to fancy up her meals.