It okay to step on the wing for as long as one don't step on movable parts like ailerons or flaps.
Only if there is sufficient structure under the skin to hold you up, kind of like walking in the attick over the ceiling joists, watch your step. Most low wing commericial aircraft, with an overwing entry have a thicker skin, painted with non-slip black coating about 1.5 to 2.0 feet wide, next to that in red letters "NO STEP". Of course larger commercial or military aircraft have heavier structure and thicker skins, as well as hatches as on the 130 there were three, to be used by ground crew, but also as escape hatches in the event of ditching in the water!