This is just my opinion based on the insights of others on the sub over the past few months.

• Just because this is a competitive gave, that doesn't go on to suggest that all players will treat the game seriously. As a mobile game I'd imagine there is a concern that 'casual gamers' will play for fun regardless of the result as opposed to having fun BY winning. It's important for players to want to win because it will in turn encourage in-app purchases.
• By having opponents taunt at you, there is an encouraged desire to win. If someone is laughing at you during the game, all you want to do is wipe the smug grin off their face. The emotes are a constant reminder that another person is kicking your ass.This has artificially induced a stake in the game.
• If you end up losing the game and they continue to mock you, from an evolutionary standpoint they are asserting dominance over you. You spam the angry face, maybe you passive aggressively wish them good luck or say 'well played', to try and play down their win and imply it was a fluke. They have beaten you, they are laughing at you and there's nothing you can do about it...
• ... or is there? Now that you have lost and have had your pride diminished, you are now determined to get better at the game. You do some theory on YouTube, spend some coins and make a meta deck.
• You win for a while, but eventually you tilt and lose all the trophies you gained. And opponents are still laughing at you. Now what?
• You decide to spend a small amount of money on some gems to speed-open some chests. This will help you level up your cards. But this fails. Now you have justified spending money on this game you can simply repeat the justification.
• You buy a super magical chest and unlock Sparky. You crusie through the Royal Arena, three towering everyone. Brain chemicals such as endorphins are surging through you and making you happy. You taunt some opponents to deflect what people have done to you and have restored your dominance.

• However, you progress to a point where people can beat Sparky with their eyes closed. You lose... and then you lose again, and again. Eventually you are back where you started. Strong emotions are being evoked causing you to buy more gems. You now have a physical stake in the game through spending real money. Every time you spend and lose you are adding to this stake, making it more and more important to spend more and more money each time in order to make the overall investment worthwhile. Eventually you find a small group of players who spend stupid amounts of money on this game, because you can't just quit after having spent $500 on a mobile game.

• Basically you only spend money on their game in you're invested in a competitive sense. The best way to generate this is to have people mock you and knock your ego to make you angry. Once you give in and spend money you have a built in justification you can simply repeat. However every purchase increases your stake and investment in the game.

• As you can see, this entire process begins with emotes. When people first called for a mute button, the response was 'we want to evoke strong emotions'. When nobody in the community bought that they introduced a 'mute button'. However, if I recall correctly, the reason they have the three dots is because they want to remind you that your opponent is a very real person. When you see someone just spamming emotes, you can still this, and there is no friendly way to spam emotes.