So, the monkey farmer in BTD6 initially was terrible. Recently, however, Ninja Kiwi has decided to temporarily increase its range in the 1.6 update.
…
-Spectre losing some attacks when in range of village fixed
-Several cash issues in deflation fixed
-Some ‘regrow farming’ methods nerfed
-Monkey Farmer range increased (Temporary change)
-Several Monkey Knowledge fixes
-Monkey Knowledge Fit Farmers now unlocks in correct order
-Monkey Knowledge Crossbow Reach fixed
…
Previously, the Monkey Farmer’s Range was absolutely terrible.
Before The Update
Here was its previous range (screenshot from THIS video @4:44 because I didn’t screenshot it before NK updated its range).
Remember that monkey farms work weirdly at this time, since they pick up the bananas after they land on the ground, rather than pick them up directly when they spawn.
After The Update
Now, here is the updated range. 
Wow! It’s so much bigger! It’s very nice. I approve of this change and I hope it stays in the game forever.
With The Monkey Knowledge
Here is the updated range WITH the bigger range monkey knowledge. Note that this costs a lot of knowledge points to get to in the first place.
Here is the monkey knowledge one again, but this time on the same map and positioning as the one without the monkey knowledge. Look at this difference.
Is it Worth It?
Note that the “powerful monkey storm” knowledge requires both paths with the arrows pointing at it. This means that to unlock the farmer knowledge, you have to unlock “Longer Boosts” and “Budget Pontoons”.
I think the 7 monkey knowledge points just might be worth this upgrade if you use farms a lot. I use farms a ton, and now have over 600,000 xp after unlocking all of the upgrades. This was actually the second monkey knowledge upgrade I went for, the first being the one that gives you 10% more cash from map clears found at the top-right of the magic section. I personally don’t find monkey knowledge to be that game-breaking, so I would say do whatever floats your boat. If you think this is worth it, go ahead and buy it. If you don’t use farmers or farms a lot, however, then I would stick with upgrading something else first.
More Facts about Farmers
In the new update, the banana farmer actually picks up bananas from when the farm spawns them. However, it’s still not entirely reliable, but it’s like 99.5% reliable. Here are some instances of where the farmer will pick up bananas. Here, it doesn’t pick up up all the bananas from the top-most farm, though it picks up some.
The banana farmer is affected by the village range, though, so adding a village will make it collect all the bananas from the top-most farm.
The Temple with support sacrifices would probably increase the farmer’s range more, but I didn’t have enough money at the time. The banana farmer isn’t recognized by the Alchemist, so its range cannot be upgraded with the alchemist.
Additionally, the farmer’s pop counter shows the amount of banana money that they’ve collected.