They're... not exactly the same kind of game, so it's hard to say? I mean, personally, I don't find Animal Crossing hard. It's probably the most chill game I've ever played.
Asking which is harder between these two games is like asking "Which is harder, Diablo III or Super Mario World?"
Animal Crossing is one of those games where you can do whatever you want.. not only one objective :l I dont necessarily consider acnl hard. Dark Souls I'd say though
I haven't played Dark Souls yet, but I'm going to go with the latter. There really isn't anything difficult about Animal Crossing; things like bells items are extremely easy to come by. Yes, soft-resetting until you get what you want isn't easy, but that's the hard thing about that game. Personally, I wish they increased the difficulty of the game, while getting rid of things you have to soft-reset to get what you want.
Animal Crossing. Even if it gives me reasons to hate it to the point that I would stop playing, there are tons of adorable things that'll make me come back and give it a chance. As for the latter, I'd just probably sell the game or keep and never touch it. That's a lot easier to do.