Disappointed to see this thread really does not look at the idea of tactical shittalk more seriously
Well played shit-talk is a legitimate mindgame, it just needs to be applied correctly.
Opponents good to shit-talk:
opponents against whom you are favored to lose (depends on the player but often times high level players feel an increased pressure to beat low-level opponents in order to reaffirm their high level status. Since Pokémon is a game of skill and luck, the better player does not always win. Losing to a "shit-talking scrub" can do a lot to a player's ability to play to their full potential)
opponents you know are emotional players (players who "go with their gut" frequently or who are known for raging/complaining are especially vulnerable to shit talk)
Essentially you are trying to create an additional and artificial layer of pressure for your opponent to deal with. If you think your opponent is susceptible to playing worse under pressure, shit-talk may exacerbate that quality. Some opponents play better with additional pressure, or simply have such good playing mentality that it does not effect their play. Save yourself embarrassment and assume your opponent will play better if you shit talk them until they demonstrate to you that they are vulnerable to it.
Good times to shit-talk:
you got lucky and it put your opponent slightly behind (your opponent will feel "robbed" and they may also underestimate you based on your readiness to brag about simply being lucky. It is always good to be underestimated.)
your opponent misclicked/realized they misplayed right after input (they already feel like shit, shit-talking in this instance can increase the chances of them going on tilt)
In a best of 3 format, you won the first game narrowly (Two ways to go about it here. You can attempt to disguise the fact that you only barely won and create the illusion that the difference in skill between you and your opponent is greater than it really is. That works best on less experienced players. Against more experienced players you may want to use your shit-talk to push the idea that you are not a very good player and you simply lucked out. Again, remember, it is always to your advantage to be underestimated.)
The worst times to shit-talk:
Because you are actually mad (if your urge to shit-talk comes from a place of emotion rather than the desire to mislead/pressure your opponent mentally then it is safe to say you are tilted and not playing to the best of your ability. Take a break.)
You just lost (just makes you look like a bitch and provides 0 tactical advantage)
You got unlucky and it put you behind (this will probably just improve your opponent's mood. It will probably give them additional confidence)
As for the actual content of shit-talk, I personally prefer brevity and sarcasm. You'd be surprised how much a simple "lol" or "nice one" can get someone in their feelings. It also gives you a certain defense against shit-talk as it will demonstrate that you are not getting emotional in response.
The most important takeaway I would say is this: don't play emotionally. Play with the goal of learning and improving rather than winning. You cannot win every game, if your goal is improvement it becomes possible for every battle, win or lose, to be a success for you. If your mentality is good, you can miss 4 stone edges in a row and move on with your life. If your mentality is great, you are immune to shit talk. If your mentality is perfect, you will never go on tilt.