First of all, the Big bang was NOT an explosion in any usual sense.
In an explosion, matter is propelled via pressure outwards into already existing space.
In the Big Bang, it was space itself that is expanding. The matter is just carried along for the ride.
Second, planets didn't arise until considerably later. The heavier elements (like carbon, oxygen, nitrogen, iron, silicon, aluminum, etc----anything past lithium in the periodic table) were formed either in the interiors of stars or when those stars exploded in supernova.
Third, gravity is something that is not as relevant on the human scale as an organizational force, but it very important on the scale of planets and above. Any distribution of gas and dust will have irregularities and gravity tends to make those irregularities larger, which causes the cloud of gas and dust to split up and condense. When this happens, spheres are the effect because that shape has the least gravitational energy of any arrangement. (this is related to the fact that, of any shape, spheres have the smallest surface area for a given volume).
For objects large enough for gravity to be important, roundness is the norm.