I think yes because having to manually go and create a room is more time consuming, but the trade off is having dead sublemmys creating dead chatrooms, particularly because domain camping on matrix is a problem. (ex, I bet #sex:matrix.org is a room on matrix with minimal/no activity)
If this was to be a feature then it should be linked to the lemmy instance itself, lemmy communities shouldn’t auto create rooms on matrix.org. Also It wouldn’t make sense to add something like this unless lemmy had deeper matrix room integration even if it was barebones.