Rotom-H
The extent to which you shouldn't use status moves on Choice sets is sometimes overstated. Even if Rotom-H was to use Specs (as it is here), Defog makes a lot of sense because Rotom-H's movepool isn't large, and it switches into setters like Ferrothorn very well. The Choice lock is less relevant after you consider that the opponent will switch to a Rotom check afterward, anyway.
But, realistically, Rotom suffers a lot from not using Heavy Duty Boots. It is a good wallbreaker, but using the standard Nasty Plot + Boots set instead will allow it much higher survivability. You should swap the item and replace Shadow Ball with Nasty Plot, as Shadow Ball isn't useful coverage for this Pokemon.
Use Discharge over Thunderbolt to spread paralysis.
Hippowdon
You may have misunderstood what makes Hippowdon good in OU. Hippowdon is good because it is one of the best ways to sponge hits from Conk, Terrakion, Zeraora, Hawlucha, etc. while also giving the ability to shut down sweepers like SD Bisharp, Mimikyu and Aegislash.
This moveset does not have enough physical bulk to achieve this role and instead invests in using mediocre coverage attacks on a Pokemon that isn't particularly powerful (nor fast). If you're looking for a Pokemon to fit the 'offensive utility' role, then you should look into Excadrill instead -- though it is not the same Pokemon as physically defensive Hippowdon.
Toxapex
Swap the ability to Regenerator.
Bisharp
Sitrus Berry is alright, but Bisharp really does appreciate a power boost to Sucker Punch (which has a relatively low base power for a main STAB). You can do this simply with Black Glasses if Bisharp's survivability is important to you; otherwise, Life Orb is a tried-and-true option for this Pokemon.
Bisharp has good synergy with Rotom-H as a wallbreaker (because Rotom removes the likes of Corviknight, Mandibuzz, Hippowdon and Ferrothorn), so it is a nice selection on this team.
Mew
Offensive Mew is meh. It's extremely easy to revenge kill using Dragapult and Dracovish, and just as easily walled due to its average stats. Not running a Psychic STAB makes this worse and means you get walled by Kommo-o and Toxapex needlessly.
I recommend replacing this Pokemon. Its best role is a set-up lead on hyper offence, and this is not a hyper offence team. Pick a wallbreaker or sweeper to use here; something that matches well against Seismitoad is good for this team, e.g. Conkeldurr. My suggestion for Whimsicott's slot will give you some speed control/ cleaning abilities.
In any case, your selection of coverage moves for Mew is very strange. This came up on your Hippo and Rotom sets too. Offensive Pokemon should be focusing on offensive coverage; what does Dark Pulse hit that you can't hit with Psychic or Fire Blast, or that your main sweeper Bisharp can't hit with Sucker Punch? I could ask the same for Shadow Ball on Rotom. Try not to use 'coverage for the sake of coverage'. Be deliberate in which moves you choose.
Whimsicott
Not a terrible pick, considering that this team is crying out for a NP Hydreigon check; but there are better Fairy-type Pokemon at your disposal.
I recommend using Scarf Togekiss here, because it can outspeed and kill Hydreigon (or cripple it, depending on your set). It also offers speed control for Dracovish (meaning you don't instantly lose to it after Toxapex dies) and to clean up after Rotom and Bisharp deal damage to the opponent's Pokemon.
Togekiss is also good at using Trick, which can help stop stall cores that give your wallbreakers trouble. This is one way you can justify using status moves on a choiced Pokemon; for example, Roost is a very useful move after Togekiss offloads its Scarf.
