Sorry to be a buzzkill. But they probably hate it.
Earlier this week, I was walking in Spitalfields, which was teeming with people as usual, and everyone's - and I mean everyone's - attention was focused on the same thing: a little dog in a bat costume. This adorable little guy was in a costume composed of heavy orange material wrapped around his little body with a big black *3D bat* hoisted up from his back. The overflowing pushy crowds seemed to halt and people instead started pointing and laughing at this cute little dog. Luckily dogs don't mind as much as humans do when people point and laugh at them, but humiliation isn't the issue here.
As adorable as this dog looked and as funny as it was, it sucked for him. He didn't want to be in this ridiculous getup. He was certainly uncomfortable, and also confused. He didn't understand why it was cute or funny. Because, newsflash, dogs don't know about Halloween. And if they did know the gist of it, they would probably hate it: they can't eat the candy, their people look different and it's disconcerting (especially if their faces are covered!), and worst of all, STRANGERS KEEP RINGING THE DOORBELL.
More importantly, when we dress up our pets in costumes, it's highly likely that this act stresses them out. Some costumes can be restrictive, inhibiting their ability to move freely. This restriction on their free movement can reduce their ability to communicate, too, especially with other animals; if a dog cannot communicate through body language, another dog might become aggressive towards him. Also, clothes and costumes can be itchy and hot! You don't want your pet to be itchy and hot and unable to do anything about it. Think of how horrible that would be for them, to want to scratch, or cool off, and not be able to. They would feel like they are being punished, and that will negatively affect their mood and behaviour. Animals aren't used to wearing clothes, let alone costumes, and this unnatural constraint on them can easily lead to anxiety. This can manifest as extra noises or grooming for cats, panting or licking their lips more for dogs, and other out of character behaviours. Too much stress, especially when it's not addressed properly, can lead to more serious health and behavioural concerns.
Aside from the serious physical health concerns, we would hope that the mental state of the animal alone would be of paramount importance to pet owners. Why would you do anything to make your pet unhappy or stressed? A good rule of thumb when it comes to whether or not you should dress up your pet is to follow the same rule for when you are thinking of pulling a prank on someone. Hopefully, we have all learned by now the paramount lesson of April Fool's Day: only pull a prank on someone if the mark will enjoy it. If the mark won't enjoy it, then why do it? It's not funny unless they will find it funny. If they won't, then you're just being a dick. Pulling pranks on people just for your own amusement is real top tier bully behaviour. If the mark won't enjoy it but you will, you're an asshole. Same for pets and costumes: If the dressing up isn't FOR THEM, then it's not a good idea. Don't be a dick to your pet.
So how do we know if pet attire is 'for them'? Guidance from the PDSA says to dress your pets only when it's necessary to keep them safe or warm. Safety concerns like walking at night could be addressed with high-vis jackets. Or, if your pet just had surgery, then they might benefit from a surgical vest (and, of course, their despised Cone of Shame). If it's a holiday, maybe a thundershirt will help keep them calm during fireworks. Maybe your pet gets cold easily and likes wearing sweaters, and so a festive sweater would be fine. These are just examples of how to start thinking 'does this help my beloved pet, or would doing this be just for me?' It's hard to see how putting them in actual costumes would ever benefit the animals.
Remember, animals are living breathing feeling creatures, not accessories. Treat them with respect.