My dark marks are getting darker. Should I use a stronger brightening serum?

Not automatically. Irritation itself can contribute to dark marks. Before increasing actives, I would ask whether new irritation, sun exposure, or ongoing acne is creating more pigment at the same time.

More brightening can backfire when the skin is irritated

I would review your sun protection, recent products, and whether the area is changing or uncomfortable before adding strength. New or changing spots should be assessed rather than assumed to be ordinary post-acne marks.