When adding a control dimension to a 2D sketch in the modeling environment Fusion wants to align with the ordinate axis (X, Y or Z) rather than parallel to the target line. This is useless in many situations because to control the length of a line segment would require two dimensions (X and Y) rather than a single dimension. Most CAD users typically prefer and angle and a length to control a line segment.
If I create a new line segment and enter the length and the angle during the creation process, the linear dimension will be aligned with the line segment. If try to add a dimension to an existing line segment, Fusion wants to give a rectilinear (X,Y) alignment to the dimension.
Suggestion - allow the user to press the CTRL key to force an aligned dimension.
When in the dimension command select the desired geometry to dimension and then right click. You will get a dialog with an aligned option.