A trip (or at least a call!) to the vet is always a good idea, but unless it is within the first few hours, with a superficial skin cut like this, especially with the triangle shape, it may be too late. Even with sutures, the skin on a cut like that often has too little blood supply to be able to repair itself. Often, a triangular flap like that will just dry up and fall off. In any case, the vet could tell you whether it's best to put some sort of ointment or antiseptic powder on it to help it heal faster.
I'm not surprised she didn't make a sounds. grooming shears are SHARP!!! I was once trimming Kodi's feet and cut MYSELF on the blade of my shears. I had no idea I'd cut myself until there was blood all over him! Then I panicked, thinking I'd cut him, only to realize I had a big slice in my thumb!
To avoid this kind of injury in the future, try to slide the teeth of a comb between the skin and the mat, then use the scissors above the comb, so the teeth of the comb shield the skin from the shears!
However, this kind of injury can happen to experienced groomers too, especially on wiggly puppies. Don't beat yourself up over it. She'll be fine!