No it’s not. I can understand if people don’t think something isn’t mainline if it’s dramatically different from the other games in the same series or if they got a different studio to develop it. Having a number or subtitle on the game’s title is not significant enough to warrant distinction. Sometimes a series will drop the number in favor of subtitles. There will be other series that will bringing in numbering when it never had it before. That’s not going to tricky stuff like flip flopping between the two and developing a sub-series within the mainline.
Disney wants to reboot the Marvel part in the VS game, imho the goal being that ultimately the “Marvel” roster fits the settings of “Infinite Comics” digital publications by Marvel.
Let’s just hope later iterations of Marvel vs Capcom stay as far as possible form the concept of the past Disney infinity games .
On a side note, Marvel and Capcom are revisiting the roots of the Vs series telling the story of how the two universes came together, so hypothetically this is a sequel to Marvel Super Heroes and a prequel to X-Men Vs Street Fighter. Not that there is a canon, nor is this KoF, but you could argue that this game isn’t a “dream match” like MvC2 perhaps is.
Edit: Lol thinking on that I guess it is a dream match as some of the characters are dead lol.