Why do the Siberian methane craters have smooth vertical walls?
Permafrost is defined as


  a ground, or medium, (including rocks, soils and organic matter) with a mean annual temperature less than 0 °C for at least 2 consecutive years.


The smooth appearance of the walls can be explained by understanding how permafrost works. Permafrost is characterized by an active layer, which is a layer normally at the surface which freeze yearly during winter and thaw during summer - this layer at Yamal can be 1-2 m deep depending on the local ground characteristics (snow depth, albedo, slope, air temperature, thermal conductivity, runoff etc).  The ground in the area is ice-rich.

Then let's assume this crater was caused by a methane bubble escaping/exploding and walls could be rough immediately following the event. By deduction, there should not be an active layer found on the walls following the explosion, as it is the first time this ground (walls) were exposed to air. During the middle of the summer, above 0°C air enters the hole due to convection. The ground is ice-rich (can be porous or invisible ice but still ice-rich) and active-layer start to form on the wall. 

Due to the near vertical orientation of the walls, saturated soil (or active layer in formation) cannot stand because as soon as it thaw to a depth exceeding its equilibrium limit, it fall at the bottom of the crater due to gravitational forces, exposing a new frozen surface (and the cycle continue until equilibrium can take a few years). Thawing sediments in ice-rich soils can reach saturation quickly, further adding to the weight and accelerating the time of the fall. Rugged and unequal surface are typically the first elements to thaw and fall.

I took this picture about an hundred km from the featured crater.


This display a very ice-rich section of the Yamal Peninsula littoral in the course of erosion. This coastal section has been eroding for decades at least. At the top the active layer is light brown, and look dry (1 m deep). Just under this top-layer, the ice is poly-genetic: ice wedge ice-rich soil and massive ice can be seen as well. The active layer cannot stand on the side for the reasons I exposed in the previous paragraph (gravity).

Side-note, the jagged edges were not initiated by anything related to a pingo. It takes decades, up to many centuries for a pingo to develop until its natural collapse. When it happen, a depression can form at its center - which could lead to a lake, but these are not overnight processes. Also, contours of the pingo mound would not appear as jagged edges if this was a pingo - the remains would appear pretty old and vegetated, alike the surrounding countryside.