Pixels are a flat grid.... think....
Each grid space is either filled or unfilled entirely. There is no way to "partially" fill a grid space. In other words, you can't have a 45° fill cutting across the middle of a grid space. It's either all or nothing.
So, with this in mind, if you were to draw a line from the top right to the bottom left grid spaces, you get a "stair-step" effect.
This is just how pixels work.
In order to make this stair-step less apparent, anti-aliasing is used. Anti-aliasing employs a method of lowering the fill opacity or tint color of surrounding pixels in an attempt to make the "steps" less visually apparent.
Without the grid lines:
(This is a manually created representation of anti-aliasing to show the theory behind it. The application algorithms to anti-alias are much better than my manual representation.)
When reduced, the anti-aliasing should make the stair-step less prominent, but it will never go away entirely:
(Again, a manual representation, applications are better at this.)
Okay, so all that being posted.....
When you rotate or skew a path and it is not at a 0°, 90°, 180°, or 270° angle (straight horizontal or vertical), you are asking the path to cross or split a pixel, which can not be done, so anti-aliasing is introduced.
Anti-aliasing is your friend, it's there to help. But it will never eliminate all indications of the pixel grid. How prominent the stair-stepping appears on your monitor is entirely dependent upon the pixel density of the monitor.
See Here: Photoshop Vectors turned into raster images automatically
and here: Is it mandatory to keep images at 72DPI for web design?
For pixel density explanations.
Specific to your issue....
To make the anti-aliasing less prominent overall on the rotated text, you can try some little tricks...
- Lower to opacity of the text a bit to 95% or 90% to help it blend in with it's surroundings.
- Add a stroke to the text which is slightly darker than the background the text is sitting on.
- Rasterize the text (or convert to smart object) and then apply a very small, slight Gaussian blur to it.
- Work larger then when saving reduce for output.
Each image is different and make or may not benefit from various blending tricks.