Johnny Cage has always been really good, people just downplay him because other characters were doing well at tournaments or sleep on him. Pretty much best rushdown in the game and a handful of bad match ups, but he's number 20 on the tier list? He doesn't even lose to all zoners, he only really loses to people like Kenshi, Reptile, Kabal,(possibly Sindel), and Freddy.
I'd say he goes even with Kitana, Sub-Zero, and Noob, he can lame it out just like Kit and Subbie, and one wrong move from Noob and Cage is in and likely not leaving till the round's over(this match is probably even at 5-5). Stryker and Jade aren't keeping him out for very long(bad match ups for them). If all zoners gave him a problem, then yes I'd agree he'd be much lower, but they really all don't, he beats quite a few of them.
Zoning's not as big of a deal as it once was...to be honest.
He goes even with a lot of top/mid characters, beats quite a lot of characters and has bad matches with a handful of characters, does this sound familiar? Because it sounds just like Kitana and Sonya who most classify as top 10 range characters.