Is a bad practice make variations on power's tracks width in pcb?
I would like to know if variation on tracks's width like the signalized in red on the figure below are very problematic.


At 10 kHz (or probably even 10 MHz), the indicated trace width variations are not significant.

It's pretty common to have this kind of feature in a design. Particularly when high currents are involved you might want the trace as wide as possible to minimize voltage drop, but need to narrow it down in certain areas to fit other traces or components. In a DC or low-frequency AC power track this will cause no problem as long as the narrow section is short (you could calculate the actual resistance effect and be sure it doesn't exceed your requirements).