I have a newbie project similar to super monkey ball, with a ball moving through landscape comprised of 3D obstacles. I plan on using Godot's 3D physics for collisions, etc... My question is, is it best to construct the level outside of Godot, then import it as a single model? Or, import the individual 3D objects that comprise the level, and build the level within Godot?

I need to tilt the whole level left/right when the player moves the controller accordingly. This is straightforward if the level is imported as one model. But if I use the other approach, and import several 3D objects that comprise the level, I'm not sure how to tilt these around the level's player's location / ball. Actually, thinking about it, regardless of the approach, I need to tilt the level from the point of the player's location.

Either solution should work, but I would recommend making your level out of pieces and putting them together in Godot. This will save you time if you can reuse the pieces in other levels, as well as improving performance if you have large levels. On a whole, single-mesh large levels are not ideal for games because it forces the game engine to draw the entire thing every time. There are, of course, exceptions to this, but starting out I would recommend the modular asset approach.

For tiling the level, what I would recommend you do is have your entire level under a single Spatial/Node3D node and then tilt that to get the rotation. That will allow you to rotate the entire level as if it is a single mesh, even though it is comprised of many. Just make sure to keep the player and any objects you may NOT want to rotate as children under a different node, not the Spatial/Node3D containing the entire level geometry and collision.

For rotating from the player's perspective, I'm not sure right off what would be the best way to approach that... I'm sure there is a good way to approach it or get something similar, since Super Monkey Ball has to use some technique, but what it is, I cannot say. I would see if you can find a reference/resource showing how to implement such a mechanic, whether it be in Godot or another game engine (Unity, Unreal, etc) that you can convert to Godot.

Though, taking a quick Google look, I'm thinking it is possible that the tilt is not, in fact, the level tilting at all, but the camera tilting and then the physics of the ball being adjusting accordingly. I have nothing to back this up, but looking at other developers implementations, I haven't found one that does the tilt and so I'm thinking it might be a visual trick rather than a mechanic. That way you don't have to worry about rotating the level at all and can get a rotation per-player (like in the multiplayer games/modes) without having to adjust the physics engine or somehow apply the rotation of multiple players all at once. Something to think about, potentially at least.