Maybe feint on Skarmory so wide guard can't block heat wave or earthquake.
Sand boosts special defense and garchomp hits physically, Snow boosts defense and delphox hits specially, rain makes rotom's thunder 100% accurate into mons weak to electric, so yea sunny day probably isn't good on whimsicott because this team doesn't look to struggle with weather much.
Darkest lariat should be swapped out for throat chop because incineroar already does nothing to defensive walls set up or not, and throat chop trades 5BP for the ability to stop your opponent from using parting shot/hyper voice/perish song.
Your team looks like it can't really deal with trick room as is. Trick room on whimsicott to reverse their trick room and Taunt on skarmory to stop trick room are your two options if you want to keep the same mons. Whimsicott is probably the better option of the two since sunny day should already be getting replaced and mental herb can block taunt. If you play well it is possible to just play through their trick room without either though, so you can keep encore whimsicott and offensive skarmory.
Overall it looks solid to me, though I'm not a pro or anything. Subsitute delphox deals with kingambit, dual wingbeat skarmory deals with sash sneasler, incineroar gives the team some longevity, you have speed controll, protect on most your mons, and you can hit weather teams for super effective damage.