So, I know this isn't really a question that can be fully answered, but it's fun to put it out there because nobody seems to notice it. I have no idea how ghosts work because I've never been one myself so anyone wanna guess how Carrie can touch things sometimes and not touch things other times. Like in the episode where gumball gets the email from the ghost and he ignores it, it turned out to be (spoiler alert if you dont catch up with gumball) Carries dad. She wasn't able to hug him until he became a ghost, ok then. So ghosts can only touch other ghosts. Wait, but how then did (again spoilers) darwin kiss carrie in The Matchmaker. Obviously there had to be some sort of physical connection there, so it doesn't really add up.
If you look more into it, I bet you'd find a million instances of her touching something that isnt a ghost and vice versa. I kinda just wanted to post this to make people really question how this works. But y'know if anyone has an actual answer to this, I'd love to hear it.