The short answer to this question is yes, opponent's mulligans can tell you something. However, things that it will tell you can vary significantly, generally based on the class your opponent is playing. Firstly, decks tend to be either oriented towards the short game, the mid game, or the long game. For example, Zoolock is one of the most well-known short game decks, whereas an example of a long game deck would be Druid ramp. Using an example of warlock, with the current meta taken into account:
Warlock is generally the easiest to discern based on mulligan. If your
opponent mulligans heavily, the most likely possibilities are:
- They are playing Handlock and don't have a good hand.
- They are playing Zoolock and started with no one-drops.
On the flip side, if they keep most, if not all their cards, the most likely possibilities are:
- They are playing Zoolock and have at least 1 or 2 one-drops, ideally an undertaker.
- They are playing Handlock, and have either a twilight drake in hand or a mountain giant, as well as one of their earlier cards or removal,
including hellfire, mortal coil, ancient watcher + sunfury protector.
Keep in mind, there are other, less played warlock decks that could throw this off, but generally, per class, there tend to be two to three decks that 50-75% of the hearthstone population is playing.
Unfortunately, it would be very difficult to break down all possible scenarios, as information that might be derived from your opponent's mulligan is dependent on a number of things. The most key things to think about when deciding what your opponent's mulligan means, other than knowing what net-decks are common in which class:
- What rank are you, and what is the 'deck to beat' at that rank?
- What class is your opponent playing?
- What class are YOU playing?