It depends on whether the planet is in the habitable zone. Many asteroid bases may not benefit greatly from sunlight and will be reliant on artificial lighting. Space might also be an issue if the surface is subject to scarifying clouds of dust, or harmful radiation due to the lack of atmosphere. Not all solar radiation is the helpful kind. I agree entirely though that the cost could be lower and that given space is less of an issue than a ship as you just need to dig for a bit longer.
The "habitable zone" is defined by average temperature, but a greenhouse allows weaker sunlight to warm things up, at least out to the fringes of that (i.e. Mars). Plants can be bred or designed to thrive under different light conditions - even in the habitable zone you'd probably need to do that a bit, since peak wavelengths vary from star to star.
Some Non-Agricultural worlds would be on the verge of or just at self-sufficiency through conventional means, too. Pop 6, Atmo 3, Hydro 3 still qualifies as Non-Agricultural, but only has millions on a planet with up to 42% of Earth's atmospheric pressure and 26-35% surface water. So you need to judge things a bit on the actual numbers, not just the Trade Codes. Non-Agricultural mostly means there's no surplus food to be found there for sale, after all.
The fallback of fusion powered hydroponics would be common on most Non-Agricultural worlds. But also provide a collapse vulnerability. Hard Times (MegaTraveller) is the best treatment of how that can go.
So, getting to your main question, it is germane to note that every Hi-Pop world is also Non-Agricultural. Billions of people take up lots of room needed to produce wheat, rice, and soybeans. It's also reasonable to note that even with a trade system as detailed as the Imperium's, it is virtually impossible to feed billions of sophonts strictly from interstellar trade. That kind of thing is more a Warhammer 40K than Traveller.
(Bold added for clarity)
Uh, no. In order to be Non-Agricultural a world needs to be Atmosphere 0-3, Hydrographics 0-3 and Population 6+.
High-Pop worlds are Population 9+; the VAST majority of Non-Agricultural worlds are not High-Pop, and very many High-Pop worlds are not Non-Agricultural.
Though some are both, for sure.