Are Boeing 737 winglets adjustable from the cockpit?
Winglets are fixed to the ends of the aircraft. As can be seen below, the winglet has no movable parts and is fixed to the end of the wings. 

http://www.sae.org/dlymagazineimages/2861_2313_ACT.jpg

Boeing 737 winglet; image from sae.org

What you'd seen is an optical illusion. The port and starboard side winglets would not be much different, usually (except that they are mirror images externally). 

Interestingly though, Boeing has filed a patent for an adjustable winglet. Airbus is also working on a similar project, called the morphlet.Both of these are geared towards reducing drag and saving fuel.