The problem with pointing out issues with the product is that you don't know the history, you don't know what they've already tried, you don't know a lot about the company. You need to be very careful in pointing out issues with the product, so it doesn't come across as someone who makes suggestions based on ignorance. A better method would be to ask questions in the interview. That could show that you've thought about the product, and want to know more about it.
This won't go over well:
Candy hairbrushes made with caramel get all droopy. I recommend you make them with a hard sugar candy. The hard sugar also allows for a lot of different colors. If you hire me, I can help make this a lot better.
This is a better approach:
I was looking at the candy hairbrushes you make and wondered why you used caramel instead of a harder sugar candy. Can you tell me more about that?
For all you know, they tried sugar candy and ran into license issues with another company. Or the caramel hairbrushes sell a lot better because they taste better. Or they've been having supplier issues. There's a lot you don't know, you can't know, and you shouldn't make recommendations until you do know more.