ArtieYes, with the right prices, it can. For example - you have a trade route A->B->A with a profit 20k per unit. Let's say you have a ship with 500 units of cargo capacity and you are able to make 4 runs in an hour, which is 40 millions credits of profit. With a fleet carrier nearby, which will provide just 17k of profit per unit, but you will be able to make 5 runs in an hour, your overall profit is 42.5 millions. Definitely interesting for the trader. When balanced on both ends (selling and buying the commodities), you as a fleet carrier owner can have a profit based on the price differences.
As you said, the spread between minimum buy and maximum sell prices must be wide enough to make this work on both ends, otherwise you'll still be doing half of the hauling between station and carrier yourself.
Let's look at silver for an example. Minimum buy price is 3,033 Cr, maximum sell price is 49,426 Cr. So the total profit per unit is 46,393 Cr. Loading silver into your carrier takes about the same amount of time as unloading it, and you want to pay someone else on either side to do it for you. This means your buy offer can't be higher, and your sell offer can't be lower than the median price of (49,426 + 3,033) / 2 = 26,230 Cr, or you will take a loss and/or do half the hauling yourself.
And now look at this from another player's perspective: You have the choice between hauling silver to a station a few systems away and make 46,393 Cr per unit, or hauling it to my carrier (or from my carrier, depending on where you are) for only half that profit. Trading with my carrier makes sense only if it cuts your travel time in half, or you will take a loss.
Due to the upside down relationship between long and short distance travel in ED, most of the time of a trade run is spent in supercruise between main star and station/settlement and on docking/undocking. Hyperjumps make up only a small fraction of total travel time unless you're flying some E-rated build. This makes it very difficult, if not impossible, to generate passive income from commodity trading with a carrier. A carrier in the middle of a trade turns a single trade run into two, and has to be competitive on both ends. I've done trade runs for a while now and have never seen an attractive trade opportunity involving a carrier on either end. So it's really just for fringe cases like the aforementioned CGs or the booze cruise with Rackham's.