No, you don't get booted back to the lobby if other people on your team were missing from the start of the match (IE, before the game timer starts only 3 or fewer squid kids are standing on spawn, and the team life HUD shows a squid outline in place of a squid icon for the missing players). The match just plays out with the uneven teams, the result counts for your vibe, and presumably also counts for splatfest results. After all, the game explicitly recognizes that the game STARTED with uneven teams but does nothing to mitigate it in the match itself or even for a player's personal statistics, so why should we expect that they don't count for splatfest results sans word from Nintendo?