I haven't tried Soraka mid, but I might take it into a Zed or the like if I was feeling confident. You won't be doing a ton of damage, but you're mostly there to keep your opponent from snowballing, transitioning into a utility mage with mild damage late game. You will have to be able to maximize damage on Q and E to really succeed, which will either call for amazing prediction or a willingness to get close to your target. Her Q lands almost instantly if you cast it next to where you are standing, so you can easily hit the center if you are near your target, and can get both hits from your E with the Q slow.
I don't think she's bad, she just doesn't fit the meta that calls for an APC in mid, since she won't really be filling that role even if you build pure damage. You should be able to find some videos of Soraka mid on Youtube if you would like to see how it is done. Notably, a lot of the videos I saw of that were made right after the rework, when E was still on a fairly short cooldown.