Converting a JPEG file to PNG does not improve the underlying image quality. It changes how the existing pixels are stored.
Why lost JPEG detail does not come back
JPEG compression can discard image information when the file is created. Once that information is gone, changing the container to PNG cannot reconstruct the original pixels. The PNG simply stores the decoded JPEG result without introducing the same kind of additional lossless compression.
Why the PNG may be larger
A photographic image contains a lot of subtle variation. PNG must preserve that variation losslessly, including any JPEG artifacts already present. As a result, the PNG can be much larger while looking essentially the same.
When conversion is still useful
You may need PNG for a software workflow, a platform requirement or subsequent editing that benefits from avoiding another JPEG save. Conversion can also be part of a process where transparency is added after the fact, although JPEG itself contains no transparency information to recover.
Better approach
If quality matters and you still have the original source image, export a fresh PNG from that source rather than converting an already compressed JPEG.