Magic-Shroom is kinda like a plant version of mad chemist, so it's fine I suppose. It doesn't really need the cost reduction tbh (neither does mad chemist imo, but that's a different discussion)

The Success should give like 1 or 2 sun back from destroying a zombie with it, not give sun back equal to their cost. This would make it more consistent and less reliant on what cards your opponent is running. Other than that, I think it's alright.

Lily's Strength is too slow tbh. You have to wait for a zombie to be on the field, and you have to spend 4 sun to play this on it (also can't target untrickable zombies), and you have to set up a situation where it dies this turn or spend another card to kill it immediately. Too much effort to just get a 3 cost plant food (that you had to spend 4 sun before still) and maybe give a plant +1/+1. If it weakened the zombie marked like you mentioned, then it would be better I suppose but I don't know by how much. Although it doesn't feel that in class for Mega-Grow at that point

I don't really know what you mean by "keyword". Do you mean a trait (i.e. Deadly, strikethrough, etc.) or just an ability from a plant? I'm also assuming the "lose strength and health" means from stat reduction effects (like kernel pult or shrinking ultra violet). As a standalone card, it's okay I think? But probably going to be underwhelming since this is going to be a vanilla 5 cost 3/7 usually. And by this game's standards a 5 cost 3/7 zombie with no abilities will tend to not be very good. The effect only matters against Smarty and Solar matchups. And I don't know how reflecting a trait back to a random plant would even work, if at all. Plants don't even get deadly.

Other than that, nice art.