That was a great match(random matchup is pretty much the same as VGC anyway. 2 on 2 Doubles on level 50) but the only reason you may find Whimsicott useless is because you're not using Prankster. Whimsicott is a support Pokemon, not an offensive sweeping kind. With Prankster, you get priority Charm, Stun Spore, Encore, and other more tricks. But if you still would like to stick with Weaville then go ahead, whatever floats your boat.
The Hydreigon sort of only won due to hax. Also, it is not really that good since the elemental fangs only have like 65 base power not to mention none receives STAB. Sure you get perfect coverage but then if you can't deal enough damage then it's quite pointless. Leave some coverage for your teammate. Hydreigon isn't the only Pokemon so having perfect coverage for the cost of power is kind of pointless. Like let's say Hydreigon with Landorus. You can make the two work together by using Earthquake and making Hydreigon just knock out the survivors. In that way, you can make both players work together. But if you want to keep it, it's fine. I'm just saying that having Hydreigon doing special moves would be more effective.
Also, I forgot to mention in my first post, your team is quite weak to Ice. Maybe Latias can take a few Ice Beams but Landorus and Thunderus can't afford to get hit by an Ice Beam due to being so frail. Terrakion can counter Ice-type threats but eventually will wear out. I'm not telling you to revise your team, just counter Ice.