Distinctions between "mainline" and "spinoff" are largely pretty arbitrary to begin with. If you asked a bunch of people to list what they consider to be mainline Mario games, you'd get a bunch of different answers.
That said, from what we've seen Let's Go has a very similar gameplay structure to the unambiguous mainline games, with the only major distinction that we know of being how catching wild Pokemon works. They're more similar to the mainline games than Colosseum/XD, which I think are the next closest things that we can compare them to. And I'll bet you anything that when Pokemon Bank releases on the Switch, probably around Gen 8's launch, you'll be able to transfer Pokemon you caught in Let's Go through it.