Anything that has a direct internet connection can be cracked. Hell, even something that only connects via a physical firewall or proxied connection can be cracked if said cracker is determined enough.
(Cracker is the correct term, hacker just refers to programmers. It's a reference to 'hacking away at your keyboard'.)
However, though malware does exist for iOS, and it's fairly plentiful, the iOS app store is vetted, just like Google Play or Amazon apps. You'd be pretty unlucky to get malware if you are careful online, don't download anything dodgy and don't download apps from outside of the app store.