Mouth != expression. The mouth of the Alien expresses nothing. It is simply a mouth. It has no eyes, or eyebows. It's mouth can't display disgust or anger or anything, because it has zero emotions. It is utterly alien to humans, and its actions are what makes it terrifying. You're massively over exaggerating what it can display.
A mouth can express many different things. See character design for games, movies and cinematography.The most popular aliens are so because they're interesting stories/design going into pop culture. Your xenomorph is one of the top aliens ever. Doesn't display jack with its face. Still scares people.
The point is a blob doesn't instill much fear due to it being so plainly designed and so limited with expressions. Action can instill fear. An alien who does such actions AND has interesting expressions is more intriguing, entertaining, sophisticated as a character.As for the Blob, it isn't just about its design, but what it instils, which is fear, and it does so without any expression or humanoid features at all, because of its actions. You keep ignoring the fact that fear and terror can come from actions, from what is utterly alien to us. Something doesn't need a face.
They do if you check their character portraits and cinematics. Their faces have multiple expressions and limited emotions, because their mouths and eyes change shape.They don't. By definition within the hive mind the zerg display zero emotions. Just because it has a face doesn't mean it's actually expressing anything. They don't display fear or anger, simply the same expression constantly, and again it's because of the fact they can kill you in horrible ways that makes them scary, nor that they have eyes (which display no emotion) or mouths (which display no emotion). The Overmind is simply a looped animation portrait that doesn't react to anything. It isn't displaying anything.
Everyone has opinions, but the most popular aliens are not faceless, emotionless blobs. The biggest alien stars such as the Xenomorph are successful because their body movements and expressions, even if they only have mouths, fit a plethora of cinematic moments.Your claim of not nearly as terrifying is utterly subjective as different people get scared by different things. A living blob is still going to kill you, and it's going to be painful. Doesn't need a face to make that scary which is my point all along. I notice that you ignored the Mike Myers and Jason point, two people in horror that scary people without expressing anything at all.
Sure a faceless blob can be scary and somewhat interesting. However, it's still a lot less entertaining to watch or interact with than a sophisticated alien with facial expressions.I get it, you don't like the design, but that's utterly different than saying that something can't be scary without a face. It can.