This is a bit of a hack, but it makes the text "at all readable".

*UPDATE* Found a better hack, see below...

1. import controlP5.*;
2. ControlP5 controlP5;
3. 
   
4. int myColorRect = 200;
5. int myColorBackground = 100;
6. PGraphics cp5;
7. 
   
8. void setup() {
9.   size(400,400,P3D);
10.   controlP5 = new ControlP5(this);
11.   controlP5.addNumberbox("numberboxA",myColorRect,100,140,100,14).setId(1);
12.   controlP5.addNumberbox("numberboxB",myColorBackground,100,180,100,14).setId(2);
13.   controlP5.addNumberbox("numberboxC",0,100,220,100,14).setId(3);
14.   controlP5.addSlider("sliderA",100,200,100,100,260,100,14).setId(4);
15.   controlP5.addTextfield("textA",100,290,100,20).setId(5);
16.   controlP5.controller("numberboxA").setMax(255);
17.   controlP5.controller("numberboxA").setMin(0);
18.   cp5 = createGraphics(width*6,height*6,JAVA2D);
19. }
20. 
    
21. void draw() {
22.   beginRecord(cp5);
23.   controlP5.draw();
24.   endRecord();
25.   background(myColorBackground);
26.   fill(myColorRect);
27.   rect(0,0,width,100);
28.   image(cp5,0,0);
29. }

I guess it depends on the particular conditions. On my computer (Processing 1.5.1 + ControlP5 v0.5.4) setting a higher resolution results in more crisp and clear text. Don't know why, cause it's also weird that it re-scales automatically. In other conditions one might need to use it with image(cp5,0,0,width,height); to scale to the window size or -as in your case- don't need a higher resolution to begin with. Of course that's the ideal, but for those still needing better quality, the upscaling options remains...

Whatever works!