When developing your Halloween yard haunt or cemetery, decide on a theme and stick with it. It'll be much more satisfying to you and your guests and trick-or-treaters if they feel that they are entering into another world. Nothing will throw that off more than if there is something there that is out of place.
For instance, if you're trying to create a cemetery or graveyard, you're going to want to load up your yard with tombstones, maybe a coffin, corpses, some ghosts, a witch or vampire, and perhaps some bones and/or body parts strewn about. You can even throw in some critters like spiders, bats, snakes, and black cats. And, some spider webs here and there would add some great detail.
But, if you drop an alien in the middle of all this, your guests will focus on that (because it's so different) and they'll spend more time trying to figure out how that fits into the scene than they will taking in all your other hard work. Stick to the plan and send the alien home.
There are plenty of great themes to chose from and have a lot of fun with pirates, cemetery/graveyard, vampire, spiders, swamp, haunted forest, alien invasion, insane asylum, etc. The list goes on and on. They can be as complex or as simple as you desire. But, whatever you chose, make it your own. Create your own collection of fun or scary decorations and props. And, stick to the plan!