I think one of the biggest issues with the "Terraria is 2-D" argument that Hypalaser brought up is the validity of such. The only time when a character should be really considered a 2-D entity is if they're specifically stated/implied to be such in this case (E.G, a character that is a program on a computer in a fictional series could be considered 2-D). Terraria doesn't imply such a statement whatsoever. If we use such logic, we can blindly state that Steve would beat Saitama because Saitama is a 2-D animation. Realistically, all programs are technically 2-D. However, since this is fiction, I'm ignoring this rule.
I'll try calculating the lifting strength for the Terrarian and Steve myself.
Steve
For this, we're going to use single gold blocks.
Blocks in Minecraft are about a meter long for every dimension, so each block is a cubic meter.
According to Google, a cubic meter of gold weighs 19.3 tons.
The player has 36 slots in their inventory (Excluding the crafting grid and armor slots, for now).
A Shulker box can hold 27 items in it, each slot containing up to 64 items.
64 * 27 = 1,728 items
Since gold blocks can be stacked up to 64, we're going to multiply the weight of a block of gold by this number.
1,728 * 19.3 = 33,350.4 tons
A player can hold up to 36 Shulker boxes, as they are not stackable.
33350.4 * 36 = 1,200,614.4 tons, very powerful if you ask me.
If you're going to ask "What about enchanted golden apples?!?!", those had their crafting recipe removed - and thus are now only found in End cities. We're using the most up-to-date version of Minecraft for such a feat, so no.
Terrarian:
We're going to have a low-ball and a high-ball for this one:
Lowball:
We're going to use thrones - items made with 30 gold bars - for the objects that the Terrarian can lift.
Ingame, the Goblin Tinkerer is 42 pixels tall - the same height as the Terrarian.
The Terrarian is 3 tiles tall.
A tile, according to the depth meter in the most up-to-date version of Terraria (Desktop), is two feet long.
2 x 3 = 6 feet tall, or 1.8288 meters.
1.8288 / 42 (Height of Goblin Tinkerer in pixels) = 0.0435428571 meters for each pixel.
When gold bars are placed, they appear as three ingots:
(This sprite is a 3x3 block of gold ingots, sorry).
Using GIMP to measure the dimensions of it:
Since the gold ingot in the inventory is depicted as a rectangular prism, I'll calculated the volume using this formula:
V = L * W * H
I decided to measure the size of a gold ingot in GIMP with the measuring tool, each line is on a different ingot for convenience:
If you can see the lines, at all:
Red line (Height) = 8 pixels.
Blue line (Length) = 16 pixels.
Green line (Width) = 8 pixels.
We first convert them into meters, then multiply them by eachother
8 x 0.0435428571 = 0.348342857 meters
16 x 0.0435428571= 0.696685714 meters
0.696685714 x 0.348342857 x 0.348342857 = 0.0845377577 cubic meters, or 84537.7577 cubic centimeters.
According to google, the density of gold is 19.3 grams/cc
19.3 x 84537.7577 = 1631578.72361 grams, or 1.798507681698879 tons per ingot
Thrones are made with 30 gold bars (And 20 silk, but that doesn't matter), as stated before.
1.798507681698879 x 30 = 53.9552305 tons per throne.
For this, we're going to use the main inventory, which is about 40 slots.
Thrones can stack up to 99.
53.9552305 x 99 = 5341.5678195 tons per stack of thrones.
40 x 5341.5678195 = 213,662.71278 tons, which is about 17.769% of what Steve can lift.
So to conclude, the lower-end lifting strength of the Terrarian is slightly below a fifth of what Steve can carry.
However, if we highball the lifting strength...
I'm assuming that a nebula fragment has the weight of a galaxy within it, given that its description states that the power of one resides within it.
We're going for a safe lowball for the weight of a galaxy. According to google, the weight of our galaxy is about 700 billion (700,000,000,000) times the mass of our sun.
The weight of our sun, according to Google, is 1.98900e30 kilograms (2.1924971974286074e+27 tons)
2.1924971974286074e+27 tons x 700,000,000,000 = 1.53474804e39 tons for each fragment.
A nebula monolith (That purple thing that pulls the moon closer to Earth ingame) is created with 15 nebula fragments, and can stack up to 999.
1.53474804e39 x 15 = 2.3021221e+40 tons for each monolith.
2.3021221e+40 x 999 = 2.29982e+43 tons for a stack of monoliths.
As stated before, there are 40 slots in the main inventory.
2.29982e+43 x 40 = 9.19928e+44 tons.
Comparing that to Steve's lifting strength...
1,200,614.4 / 9.19928e+44 = 1.3051178e-39
1.3051178e-39 x 100 = 1.3051178e-37.
Steve's feat of lifting 36 shulker boxes full of 1,728 gold blocks each is absolutely nothing compared to the Terrarian's feat of lifting about 39,960 monoliths that are individually (at the least) 15x heavier than the milky way galaxy.
That is, if we wank the Terrarian's feats.
If you didn't know, there IS a character in Terraria that has a better lifting strength than both the Terrarian (Using the more logical lowball) and Steve.
Wanna guess?
No?
The celestial towers.
The celestial towers have the ability to pull the moon closer to earth with a fraction of their power.
The moon has an approximate mass of 7.3476731e+22 KG (80994231671048617984 tons)
TL;DR, the Celestial Towers are stronger than both characters. But since the Terrarian sorta scales to the Celestial Towers due to, y'know, defeating them (Along with the moon lord, who can be deemed stronger than the Celestial Towers), it can be assumed that the Terrarian is technically stronger than Steve, anyways.