Don't mistake popularity for something being 'good' or a judge on if something is worth doing. There are plenty of examples in which the popular opinion is not representing reality.
People of the earth used to believe the Earth was the center of the universe. Just because it's a popular belief doesn't make it right or good.
You example doesn't really make any sense though, since one is a fact - Earth is not the center of the universe - and the other is a preference. You can never really judge entertainment 100% objectively, since peoples preferences differ. Example: no matter how great reviews and how popular a first person shooter game were, I would never play it because I don't enjoy that type of games.
The reality is that lots of people are playing Battlegrounds, otherwise the queue time would be much longer. Presumably, people wouldn't play something they didn't enjoy, so we can probably conclude that a fair amount of people thinks Battlegrounds is a fun mode to play. I would argue that Hearthstone's main objective is to provide fun and entertainment, and the large playerbase would indicate success in that regard. Doesn't that make the mode good? I think it does. That doesn't mean that Battlegrounds can't be made even better (it is still beta after all), but the core idea and gameplay works. Not everyone will enjoy it, of course. It is literally impossible to make something every living being will enjoy.
I think the critique that Battlegrounds is a 'dumbed down' auto battler is incorrect. There may not be an interest system, but sequencing matters much more, and while there of course is a lot of attack RNG, you can still lose a game because of wrong positioning. There's plenty of RNG in TfT too (which minion will recieve a random armour buff, which of three possible minions will clone themselves etc.). It's different because it is based on a card game rather than a moba, which makes different mechanics matter more, and I feel that people used to playing Tft and Autochess doesn't fully recognize this.