Teenagers from Outer Space does not pretend to be anything more than what it is: science fiction camp with a sense of humour. In this respect, the film actually works and as a consequence is rather entertaining.
In fact, the premise of the film is actually plausible. If there was such an alien race from outer space, why wouldn't they see earth as a potential grazing ground for their monstrous cattle, and why would they necessarily see any value to human life apart from any other life form.
However, the film recognizes its low budget, and instead has a lot of camp fun playing out the scenario of good alien and bad alien. We have good alien Derek, who admires humanity and ends up not only falling in love with an earthling, but wanting to make his permanent home on earth. Then you have the bad alien, Thor, who enjoys shooting humans like hunters shooting ducks out of the sky. In fact, in a morbid kind of way, it is actually humorous in seeing him zap anything he can into a cheap looking skeleton.
If you like the corny camp science fiction films of the 1950's, you'll enjoy Teenagers from Outer Space.