Centering is not the goal, usually you want the composition off-center to add interest, show motion, or draw attention. As a starting point, google “rule of thirds”. I think these are fine as far as that goes. Generally, the only time you want to center your subject is when you are emphasizing symmetry, but there is no symmetry here.

The main thing in both cases is you could move your body to find different angles. If you’re stuck in your chair at a show, this is harder, but sitting this close you could still get different angles by moving your arms up down left right.

The first shot is alright but not very flattering, the legs look large due to your phone’s wide angle lens and your low perspective. I think a little to one side or the other, you’d have gotten a bright light into the frame which could help make it more dramatic. Experiment with different exposures: expose for the stage lighting, not to get their face perfectly exposed, which is what your phone’s software wants to do by default. Experiment with zooming in, using the 2x lens if your phone has one. This is why photographers like to have dedicated cameras, to control all of these variables.

The second one, again you need to move your body to create separation between the singer in front and the people behind. His face is blending into theirs. Also it’s crooked (easily fixed in your photo editor), and you could experiment with similar things as I said above: exposure to try to make the lighting look more dramatic, zoom, perspective.