No, a thrown net cannot be difficult terrain RAW
There is no RAW ruling on this that I can find. Certainly there is nothing in the description of the net item that would suggest that it has this effect.
But adventurers often face dense forests, deep swamps, rubble-filled ruins, steep mountains, and ice-covered ground—all considered difficult terrain.
From the PHB description of difficult terrain, it seem like only major terrain features should really qualify. I'm not sure if I would say walking across a net would be comparable to major land features even such as rubble. Simply put, difficult terrain is a term that is defined such that I don't think a net could ever qualify.
In fact, even ball bearings and caltrops are not actually considered difficult terrain. And both of these items are INTENDED in game to serve as movement obstacles/battlefield control.
As an action, you can spill these tiny metal balls from their pouch to cover a level, square area that is 10 feet on a side. A creature moving across the covered area must succeed on a DC 10 Dexterity saving throw or fall prone. A creature moving through the area at half speed doesn’t need to make the save.
Any creature that enters the area must succeed on a DC 15 Dexterity saving throw or stop moving and take 1 piercing damage. Until the creature regains at least 1 hit point, its walking speed is reduced by 10 feet. A creature moving through the area at half speed doesn't need to make the saving throw.
Difficult terrain forces you to move at half speed, but ball bearing allow you to move at full speed, but at the risk of falling. A very nuanced difference sure, but still significant I feel. For example, any abilities that interact with difficult terrain should not apply to ball bearings or caltrops nor, I feel, to nets on the ground.
Reasonable house rule
However, I do not think it would be unreasonable for a DM to rule that it had effects similar to ball bearings or caltrops in that someone can try to move full speed through but risk getting tangled or falling.
These are not difficult terrain effects however, so the literal answer to your question remains "No".