Is there a way to adjust the line weight of the hatch/pattern in Layout?
Not within LO. The hatch patterns are PNG images. You can create new ones with heavier lines. Depending on the hatch pattern you are thinking of, you might find it possible to edit a copy of an existing pattern to use as a new one.
Thank you!
Adding to @DaveR’s response, the easiest way to create hatch patterns is to knock them up in LayOut itself! You can map them out on a square page, adjust lineweights (even do multiple weights) and export as PNG!