As a lover of art and a booster of cinema, I find value judgments like “good” and “bad” to be rather unhelpful and the sign of a dilettante or a critic.
That said, Manos is a bad movie. 
It’s hard on the ears. It’s hard on the eyes. It’s hard on the brain. It’s hard on the butt. It’s exploitation that even fails to exploit.
To the extent that the word “bad” has meaning for a work of art, we can apply it here. This is also true of Ed Wood, whom I love. Could you say to an average joe, if they asked you about Manos (or Plan 9, for that matter) that “This is not a bad movie,” and they went off and watched it, do you think they would look to you for further movie advice?
We’re in a kind of rarified space here in that we know how bad “bad” can get. We know there’s a scale below which normal human experience is barely cognizant. And while we might debate where the bottom of that scale is, we all know, when we’re down there, that Ed Wood’s charming quirks seem positively witty and delightful.
If you’re standing on a Feeders or a Things and look up, you might indeed see Manos. Where any movie might end up on this scale is of course entirely subjective, as we see in Worst movie you saw in the theater.
And all that said, to call Manos a “decent” movie, simply because you’re aware of the depths to which film can sink, calls into question what “decent” means. Many shots are out of focus, almost all are badly framed, the meagre plot is confusing, the sound is punishing both in terms of what it does to the actor’s voices and its mangling of the soundtrack (which, in fairness, may be an artifact of film deterioration), and the editing feels like it’s driven more by the need to pad the film than anything.
But if you simply mean “there is much, much worse”, that would be hard to disagree with.