This is the 1346th most frequent Dutch word.
plat
Flat.
In this sentence, 'plat' is used as an adjective describing the flatness or level characteristic of the terrain.
Het landschap is hier erg plat.
The landscape here is very flat.
Here, 'plat' functions as an adjective implying that the joke lacked depth or sophistication.
Zijn grap was een beetje plat.
His joke was a bit dull.
In this context, 'plat' refers to the dialectal nature of her spoken language, often indicating a strong regional accent.