Doesn't matter if it's "skilled" or not, you're still paying someone to do something for you. And if it was trivial, you wouldn't be paying them.
At a restaurant I'm paying the chef and waiters for making me food, no matter what the quality, or if I could make better or not -- because I didn't want to cook, and they did it for me.
That alone is worth paying someone and thanking them.
The expected tip should be included within the price so the workers are paid fairly by default. Tipping can still exist to show appreciation, but it needs to have a maximum. $1-2 per individual, or $5 combined. They should be extra in every sense of the word. I make sure to tip at least $0.50-$1.00 for perhaps a $2.50 drink at a local coffee shop because I absolutely love that place. I'm tipping to reflect that they're my favorite. That's what it should be like.
It's not a matter of the work being trivial or not. You're mainly paying for someone's time. The labor itself is extra on top of that. We need to work to put food on the table and have clothing/shelter. If you're spending your time doing work for someone else, then you can't spend that time on necessities, which means your employer has to provide it through your pay.