Evil? No he doesn't do anything to damage the Valley or anyone who lives in it. He doesn't interfere with anyone's life or try to alter their decisions. Even our Grandpa was friends with him at one point.
However he's no saint. He clearly enjoys playing games with you, leading you on long quests to see how far you can be tempted to your possible destruction in Skull Cavern. You could frame this as a rite of passage he thinks the Farmer is capable of doing, or you could frame it as some guy getting his jollies watching the Farmer stumble and fail so many times.
He's also running a Casino so is not bothered by taking advantage of people by exploiting a weakness for gambling. However he hardly lets anyone in there so perhaps he knows to keep people out who hurt themselves. And once you play long enough, you'll find at least the 1-armed bandits are really 1-armed Robin Hoods, eventually giving you more Qi Coins than you started with.
Ah but the Qi Coins, Mr. Qi's captive currency. You can't use his in-Casino money anywhere else and he takes your regular g in the first place to get you in the door. If you're patient and have a Deconstructor, you can realize some G profits but he knows that procedure is a pain in the butt. You're really there to trade his worthless coins at his company store for a few Warp Totems, Hardwood Fences or Magnet Lures. And only a few nowadays, he used to allow infinite trades but no more abusing that loophole!
And then there's the Walnut Room, where he works in cahoots with the Gangster Parrots on Ginger, both getting you to solve puzzles and dig through the sand for them because they're too lazy to do it themselves. Once opened though the challenges are just starting, though some are quite benevolent if insidious, like Qi's Kindness. Balanced with things like 50K in Junimo Kart or Qi's Crop of course. At least this time Qi gives you a few more interesting items to buy using his second captive currency.
He's really the world's most understandable Bond Villain. Not evil like from a movie, instead differently motivated. A few Secret Lairs, one on a remote island. Strange dress sense. Convoluted plans. Abusing and exaggerating economic concepts. Bolstering the Farmer's capability to make them a more 'worthy' adversary. Playing the Farmer like a puppet, while treating them superficially like a friend.
I fully expect the next Bond movie to feature a rather familiar sparkly bespectacled face.