Does altitude affect how you interpret the attitude of the aircraft?
Yes, due to the curvature of the earth.  But you'd have just as much variation with a small change in pilot eye position within the aircraft.  The exact "sight picture" of the top of the nose (or the top of the instrument panel) relative to the horizon is not really something that you memorize and use as an absolute reference point with the expectation that it will be exactly the same every time you fly a given airplane.  It's more just something that you keep an eye on to watch for changes.  By the time you've climbed to altitude, you've had plenty of time to (perhaps unconsciously) get used to the resulting slight change in the sight picture between the nose and the horizon line.  All the while, other instruments such as the altimeter or airspeed indicator or vertical speed indicator help tell you whether the place you are putting the nose relative to the horizon, is really the correct place to make the airplane do what you want it to do.  Of course, there many instances where the horizon is not clearly discernible anyway.