It's not just that it's a grass type. It's the allotment of stats. Most of the time, grass type moves are Special rather than Physical, where it has a base stat 110 as opposed to 60. Although it can hit fast most of the time because of superior speed, it also doesn't hit hard because, unfortunately, it's special is low. Looking at the movepool you may argue that there are 2 physical grass type moves that it can learn; razor leaf and leaf blade. Razor leaf is ineffective therefore leaf blade is the only one that really works and that's learned at lv 45. Leaf blade has 15pp, so it's going to go down fast if you're on victory road or something. Even then that's not taking into consideration that with any of it's 5 weaknesses, it's absolutely useless because Leaf Blade has a disadvantage. Yes, you can use normal type moves, and it does have good ATK/DEF, but let's say that you go up against an ice type. Ice types usually use Specials, and Leafeon's base is 65, so it'll land a quick half-damage hit and then die because of type and stat disadvantage. Fire types are the saving grace because they use physical but it's still a weakness and Fire types hit hard. ATK+DEF was a bad pairing for a grass type
