I haven't actually tried this since I have yet to make a retro pixel style game but try setting the pixel rounding under project settings to on. Something else to try is changing sampling to point rather than linear under configuration settings.
From the manual:
By default Construct 2 objects can be drawn at sub-pixel positions, e.g. (100.3, 200.8). If the browser is using linear filtering, this can make fine pixel art appear blurry. If Pixel rounding is set to On, Construct 2 objects round their position to a whole number before drawing, e.g. (100, 201). This prevents any blurring, and can also prevent "seams" appearing on grids of objects. Note this does not affect their actual X and Y co-ordinates, which can still be between pixels - it only affects where they are drawn on the screen.