Are cracks on exterior walls serious and can I fix them myself?
From the pictures it looks like the cracks may only be in the stucco, and the underlying structural wall may be unaffected.  It's hard to tell without actually inspecting the wall, but my guess is that these cracks are only skin deep.  However, you will want to treat them to prevent them from spreading.

Here is a good article about Cracks in stucco. 

Here are some excerpts from the article.


  All buildings are subjected to a
  variety of stresses in various forms
  and this stress is often transferred
  to the plaster stucco membrane. 
  
  Stucco
  is not and never was intended to be a
  structural material. 
  
  Cement stucco is
  a relatively thin, brittle shell over
  a framed structure and can crack if
  the stresses are transferred and
  overcome the strength of the plaster
  membrane.
  
  SHRINKAGE STRESS 
  
  All Portland cement
  shrinks as it cures. Controlling and
  minimizing that shrinkage is critical
  to minimizing cracks 
  
  Shrinkage cracking
  tends to have a map-like cracking
  pattern. 
  
  OUTSIDE SOURCES 
  
  Structural loads,
  lateral force racking, thermal
  changes, wind, lumber shrinkage and
  seismic events are all examples of
  outside sources of stress. 
  
  HOW HARMFUL IS A HAIRLINE CRACK? 
  
  a hairline
  crack does not automatically mean the
  stucco is faulty, failing or will not
  function for the service life of the
  building. 
  
  MAKING REPAIRS 
  
  Once the decision is made to make
  repairs, there are several options.
  
  Option 1: Treat the joint with the
  same finish material. 
  
  Option 2: Paint the wall or ceiling
  after treating the crack with an
  acrylic paint or paint designed for
  stucco. 
  
  Option 3: An elastomeric paint
  coating. 
  
  Option 4: Recoat the wall from
  architectural break to architectural
  break with the same finish material.
  
  Option 5: A new basecoat and mesh over
  the wall area.