Why are plupples so dumb?
I spent way to long doing research on this theory, so please enjoy! I would love to discuss this further, so if you have any thoughts on this please add them.
There are 4 notable examples of plupple speech and intelligence that I could find in the show.
The first of these examples is Farch, the plupple who Garce uses as an example of plupple not being able to lie. Farch can speak in full grammaticality coherent sentences, but is still willing to kill himself for Garce, saying only "all right, but you owe me." Implying that either plupples can come back after being exploded, or that Farch was unable to make the connection that after he exploded, he would be dead.
The second notable example is Smeach, the plupple that Dr. Goondis brings to the 0-nion station, who proves only to be able to communicate with the word "plup". This seems standard among all the other plupples we meet.
Another, less obvious example, are the two plupples that discover the starship minestrone after it crashes. These plupples seem to be able to communicate in rudimentary sentences, although with poor grammar and a very small vocabulary. Both plupples prove to have very little self-preservation, both exploding themselves almost immediately.
The fourth example I could find is Moofus, the giant plupple that Maizey fought. Moofus only repeats what is said to him, with one notable exception: when Maizey makes him lie, he says "Thank you, Maizey" which shows some level of intelligence beyond that of an average plupple.
My theory as to the cause of this is the lack of education on tomat-0. This is likely caused by Dr. Goondis' tyranny. Garce even says "They’re dummies. And they’re all marching to the beat of ol’ Doctor Goondis!" So, I think that plupples are capable of higher intelligence, as shown by Farch's speech ability and Moofus' situational awareness, but are unable to achieve this because of the sorry state of education on tomat-0.