I don't think the general gameplay has a steep learning curve. But many specific details of the gameplay do. (E.g., making stairs work, figuring out pumps, effectively managing resources with stockpiles and orders) And the interface definitely has a steep learning curve, both in terms of user input to the game, and game feedback to the user.
Fixing the latter groups without significantly altering the general gameplay could go a long way to reducing the learning curve while not really dumbing down the game at all. I hope people don't get sucked into the notion that complexity for complexity's sake is good. Complexity for gameplay's sake can be good, but needlessly making something more complex than it needs to be (or allowing an early and complex design to remain complex merely out of a desire to avoid simplifying it) is not something I am a fan of.
An example of dumbing down the core gameplay would be reducing the varieties of food to generic plant, generic meat, and generic prepared meal. An example of how the game is already dumbed down (albeit simply because it hasn't gotten further attention yet) is that a dwarf's mood is completely linear.
Redesigning stairs to be simpler to grasp and to dig or build would not really affect the meat of the gameplay. Making decisions about the layout of a fortress would remain the same. The functioning of stairs once built would remain the same.
Similarly, redesigning the whole zone/stockpile thing as well as the interface for creating regions (zones, stockpiles, rooms, farm plots, et cetera) could reduce the learning curve a lot while maintaining the interesting aspects of managing resources and dwarves efficiently: One would still need to choose which items get stored where, the locations where dwarves get to perform certain activities, and so on. It would merely be easier to grasp how these concepts work, and easier to actually implement some design once that design is chosen.