I do think a lot of people are playing Corn right now because some of the other classes feel useless to them... Corn definitely isn't that good though, not to warrant his use by everyone on the plant side. At least not by my standards. The nerf to Citrons damage, and general lack of damaging abilities discourages people from using him that much. Rose is the same, her abilities are fine but her nerf and lackluster damage now really discourage people from playing as her.
So you see a lot of Corn. But again, I personally don't find him all that playable. He really doesn't have anything that special. He just so happens to be the only plant with 3 attack abilities, whereas a lot of the other plants are support abilities, and some of them are very useless, like the cactus' tallnuts. They aren't completely useless but I mean they're aren't exactly good either.
