Grass typing is weak to Bug, Fire, Ice, Flying. ALL of these are prevalent in the current meta-game, both in Smogon tiers and the Pokemon Company Online. Malamar is weak to Fairy and Bug. Less to worry about considering Talonflame is everywhere in both, Scizor is always lurking on the next team to come, and Ice coverage is not hard to throw onto a water type.
Fairy is not nearly as common despite being a new typing, and bug is shared. Switching in Malamar is an asinine strategy for a 'mon whom requires support as-is, not one who gives it. I also wouldn't consider resistance to Water *key*. A lot of types are decently common on teams already that resist it, and adding another is just fluffing your team to be anti-rain. Resistance to ground is nice, and is why considering a grass type at all on a team is always worthwhile, but it shouldn't be a selling point on something that is as gimmicky as "If I use this move once, I may be able to cover the typing they are switching in with one of my other possible 2 attacks since I'm obsessed with glare for support". Malamar is, given, gimmicky as well. But that's why he doesn't have a thread, and for another time perhaps.