There is no official statistic for the percentage of Washington, D.C. visitors who see the Washington Monument. The National Park Service tracks Monument visitors (~600k–800k/yr pre-pandemic, ~400k–600k post-reopening) and total D.C. visitors (~20–25M/yr), but not the overlap.
If 600k Monument visitors / 22M D.C. visitors = **~2.7%**. But that's ticketed interior visits. "Seeing" it (exterior, from Mall, from afar) probably >80%. The Monument is the Mall's vertical anchor—you don't visit the Mall without seeing it.
National Mall total visits: ~25–30M/yr (NPS). Monument interior: ~0.5–0.8M. Ratio: 2–3%. But "seeing" ≠ "entering."
For your purposes: **Nearly everyone who visits the National Mall sees the Washington Monument. Far fewer go inside.**