If that washer goes between outer basket and inner hub, I daresay it may well need to be an EXACT thickness as it sets the clearance and spacing of the clutch plates and inner and outer hubs. The conical spring washer is in a different location, closer to outside of clutch assembly and just under the 4 bolt clutch lifter plate.
The 25 mm. spline washer may well not need splines, it would turn of its' own to lube if something found to go in place of the OEM part, but it MUST be the thickness required there and DEAD FLAT on both sides which commonly a hardware store washer is not. A flat file can be used to flatten any dings in a hardware store washer and to remove any outside edge punchout knarf off it when it was punched out of sheet but you need that thickness. 25 mm. is almost 1 inch FYI. While that's close, the 25 mm. number is less and at the difference in mm. and inch has 1 inch too big at .018" too loose. I bring that up because the spline forces the OEM part to spin so it does not wear the shaft inside it. Spinning free, the washer will live but at a different rate of spin will begin to wear into the input trans shaft, the looser the center hole is the more that will happen when the washer wobbles.The OD of washer needs to be such that it fits cleanly inside any pocket there as well, that may not be with a hardware washer.
If washer too thin the clutch will bind and drag, the inner and outer then interfere with each other. Correct thickness would have the inner bolted down to outer basket with the outer nut and conical washer and no clutch discs installed, when tight both shell and inner must rotate freely with no drag and no excess play. A wee bit is OK.
It can work but all the detail work needed screams to find the correct part, it's so much easier. One of those 'minor' things that at face value looks OK but can make for problems in the future.