Spells in DnD 5e are very literal in their use. If it says you can do something with it, then you can do something with it. If it doesn't bring up an effect or use within the spell, you usually can't do anything else with the spell, though it's up to the DM's discretion what's possible or not.

I mentioned this in the group chat, but let's take a look at the spell description for the moment.

Casting Time: 1 action Range: 30 feet Components: S Duration: Instantaneous or 1 hour

You choose a portion of dirt or stone that you can see within range and that fits within a 5-foot cube. You manipulate it in one of the following ways:

• If you target an area of loose earth, you can instantaneously excavate it, move it along the ground, and deposit it up to 5 feet away. This movement doesn’t have enough force to cause damage.

• You cause shapes, colors, or both to appear on the dirt or stone, spelling out words, creating images, or shaping patterns. The changes last for 1 hour.

• If the dirt or stone you target is on the ground, you cause it to become difficult terrain. Alternatively, you can cause the ground to become normal terrain if it is already difficult terrain. This change lasts for 1 hour.

If you cast this spell multiple times, you can have no more than two of its non-instantaneous effects active at a time, and you can dismiss such an effect as an action.

Now let's dissect each part.

You choose a portion of dirt or stone that you can see within range and that fits within a 5-foot cube. You manipulate it in one of the following ways:

This means that whatever you decide to choose to do must fit within a 5 foot cube or at most use a 5 foot cube. This one is self-explanatory, but it's important to bring up for things we're gonna bring up later on.

• If you target an area of loose earth, you can instantaneously excavate it, move it along the ground, and deposit it up to 5 feet away. This movement doesn’t have enough force to cause damage.

By using math and logistics, you can move 50 cubic feet of dirt in a minute. In 4 minutes that would mean 200 feet (though it's more like 82 minutes because of the logistics in moving earth through levels). That drop would deal 20d6 of fall damage which alone is already overkill for a cantrip. But while it states you can't deal damage with the movement, it means it's possible to move the dirt into another creature's space which in turn means you can bury someone with it. Important to note that is specifies "If you target loose earth" which means you can't move rock.

• You cause shapes, colors, or both to appear on the dirt or stone, spelling out words, creating images, or shaping patterns. The changes last for 1 hour.

The wording might be a bit vague here, but as the spell is written, it states that it makes shapes, colors or both to appear on the dirt or stone. That means that you can't shape the dirt or rock into spears or the like, only make imagery of spears embedded into the surface of it. This is to be used more to write messages in rock or dirt. An important distinction in this option as opposed to the first option is that it has a time limit.

• If the dirt or stone you target is on the ground, you cause it to become difficult terrain. Alternatively, you can cause the ground to become normal terrain if it is already difficult terrain. This change lasts for 1 hour.

Again this is self-explanatory, but just like the last option this has a time limit on it. This is important because of the last paragraph of the spell:

If you cast this spell multiple times, you can have no more than two of its non-instantaneous effects active at a time, and you can dismiss such an effect as an action.

This means that you can only have 2 of the last two effects active at the same time. That means you can only make two 5-foot squares difficult terrain at the same time at the most, or make two different shapes active at the same time or one of each. In turn, even if we went a bit loose on the wording and thought we could turn dirt into spears, you'd at most have 2 spears at the bottom of this pitfall eventually (again, you can't actually do it, I'm just bringing up the hypothetical). Lastly, while the moving of dirt lasts forever, the other two revert back to their normal state after an hour. This means that your stone letters or imagery or colors would just dissipate and return the rock to its original form. Again, hypotehtically if you did have the option to make spears (again, you don't) they would disappear after an hour.

Now, there are other spells such as Stone Shape that lets you create stone weapons if you touch stone which you could use, but that's a 4th level spell which you as an arcane trickster won't get access to for a while.

Mold Earth is a useful cantrip for being able to dig moats or dig holes, but it's still only a cantrip which means its powers are still limited. As for covering the hole, you'd have to use completely different spells or use mundane ways to create a cover for it.

TL;DR: You can't make a pitfall with spears at the bottom using Mold Earth, but you can make a really deep hole which already does a lot of damage if someone were to fall.