Usually not fully. In very small phone cameras, the lens and sensor are a system, and the optics often become the limiting factor before a 20MP tiny sensor can be used to its full theoretical resolution.

Real lenses are not diffraction-limited across the frame, especially at the very small f-numbers used in phones. Aberrations, focus tolerance, and sensor effects all reduce actual sharpness. Even if pixel pitch is around 1µm, the optical spot is often larger than a pixel, so adding more pixels gives diminishing returns rather than linearly more real detail.

That does not mean extra megapixels are useless. Oversampling can still help with sharpening, demosaicing, noise reduction, and making fine detail appear clearer in good light. But it usually does not mean the lens is truly resolving 20MP worth of distinct detail.

A practical way to check is to photograph a resolution test chart and inspect fine line pairs at 100%. If very fine lines lose contrast or blur together, the optics are not matching the sensor resolution.

So: higher MP on a phone can help somewhat, but in many cases the lens cannot fully support the sensor’s nominal resolution.