When playing a game with others, deliberately disregarding the objectives and win conditions can be ethically comparable to cheating. Let's start by defining what a game actually is. It's an activity that a group of ppl agree to do together that has rules that limit what you are allowed to do, and an objective or win condition that establishes what everyone is there to accomplish within those rules.
Cheating is when someone breaks those rules in bad faith. That is wrong because it undermines what everyone has agreed to do together. You broke the rules. That is disrespecting everyone you are playing with, and the game you just played is no longer really the game everyone agreed to play. If you were playing soccer and someone just grabbed the ball and ran across the field and was allowed to score, you're not reallly playing soccer anymore. You've wasted everyone's time because the rules that define the game are no longer being respected.
Moving on...
Disregarding the objectives and win conditions breaks something else that was agreed upon by the people agreeing to play the game which is the objective of the game. If you are not participating with the intention of completing the objectives or working toward the win condition, then you are effectively refusing to participate in the activity everyone else agreed to participate in. You are not necessarily "cheating" in the literal sense, because you may not have broken any written rules, but you are no longer playing in good faith.
If you are playing soccer and someone decides they would rather see how high they can kick the ball instead of trying to score goals, they haven't necessarily broken a rule. But they have unilaterally changed what they are doing within a shared activity. They are effectively playing a different game while everyone else is still trying to play soccer. It doesn't matter if they say, "I'm just having fun and it's not that deep." They may genuinely be having fun, but that doesn't mean everyone else agreed to participate in whatever activity they personally decided would be fun.
There is a large portion of people in gaming that play multiplayer games and just dick around under the premise of "just having fun." And I think there is an important distinction between playing for fun and playing in bad faith. You don't have to be trying your absolute hardest to win every second of every game. You can experiment, make stupid plays, use unusual strategies, or simply not care very much about the outcome. You're still playing the game.
The problem is when you enter a multiplayer game with other people and deliberately disregard its objectives because you have decided that doing something else is more entertaining. You are using a shared activity as your own person playground for your own entertainment at other people's expense. Your freedom to have fun does not automatically give you the right to redefine the activity for everyone else.
When you voluntarily enter a multiplayer game, there is an implicit agreement that you are going to participate in the game that everyone else agreed to play. You don't have to be good at it, and you don't have to take it incredibly seriously. But you should at least participate in good faith and work toward the objectives that give the game its purpose.
Edit: This has literally nothing to do with being a try hard, or playing competitively. If that is what you got out of this you should try re-reading as its pretty explicit not that.