Final answer:
Grumio's intelligence is questioned because he is labeled as a poet, which the character G. implies is near foolishness, and there's a discussion on the ineffective use of intelligence among the characters that may reflect on Grumio's intellect.
Step-by-step explanation:
The implication that Grumio is not very intelligent is drawn from a line where it's stated, "Not altogether a fool," said G., "but then he's a poet, which I take to be only one remove from a fool."
This suggests that while Grumio might not be completely devoid of sense, he's perceived to be on the edge of foolishness by virtue of his being a poet.
The quote insinuates a stereotype that poets are closely associated with fools due to possibly their idealistic or impractical approaches to life.
In the second reference, the characters discuss the use of their intelligence, or lack thereof, to relate with each other and the world, hinting at a struggle to use one's intellect effectively, which may cast doubt on Grumio's intelligence.