Well it depends.
In friendlies my mindset is mostly "test this, try this" and basically get warmed up. I'll often do stupid stuff and stuff I don't see in tournament to see how loose I am mentally and how on point technically I am.
In tournament my mindset is always "how do I get rid of that stock/make it easier to KO them." I'm always thinking of ways to efficiently do that with minimal damage.
That means I am:
-Analyzing every player's tendencies.
-Looking out for their strengths, and ways to make those less meaningful.
-Looking for weaknesses, and ways to exploit them.
-Trying to find various parts of the stages that players are uncomfortable with or that I'm better at and bringing the match there (Randall though).
-Seeing where their tech skill is on and off, and doing the same for myself.
-Trying to react to what damage percent each player is at.
-Knowing how well I am technically and using it to the fullest extent of my abilities at the time.
-Figuring out a way to disrupt my opponents to further my chances of winning no matter what it is, and then doing everything I can to do it.