I think if you use a very flat paint like a blackboard or greenboard paint (for painting classroom chaulk boards), then the very flat surface will provide good friction between the table and the ball. You may, get more sliding of the ball if you clear coat over that flat green paint. Also, if you put a spin on the ball, it may not bounce off the table at an angle as intended because of the sliding and less friction between the ball and a rough or smooth surface. I'd leave that paint flat green.
Also, wipe your ping pong table paint with a damp sponge. If it turns a darker green, ask yourself if you like that colour. Cuz, if you put a clear coat on it, your ping pong table is going to be that same colour permanently.
The reason why also explains why your blue jeans are darker when they're wet, and that's because the refractive index of a solid like cotton is more similar to a liquid like water than it is to a gas like air. Thus, light bends less when it passes from cotton into water (or vice versa) then it does when it passes through cotton into air (and vice versa). As a result, light travels a straightER path through wet cotton than it does dry cotton, and that means it tends to enter the wet cotton and keep going in a straight direction into the cotton where it's absorbed. Less of the incident light is refracted and reflected back to your eye, and your eye sees less light as the colour "dark".
Exactly the same thing happens if you put a clear coat over a porous paint. Sure, you'll get some light reflected off the top surface of the clear coat (giving you a shiny surface), but once into that clear coat the light will continue in a relatively straight line because of the relatively small difference in refractive index between a solid alkyd paint and a solid acrylic clear coat. Thus, your incident light will enter your ping pong table and be absorbed inside it. So, you'll have a glossy surface over a darker green ping pong table. Or, it'll look very similar to the way it would if you hosed down your ping pong table with a garden hose; dark and with a shiny "wet look" gloss on it's surface.
Why not just paint your table with greenboard paint, leave it unfinished and use a damp Magic Eraser (pronounced BASF "Basotect" foam) to clean it?
A Magic Eraser when viewed under a microscope has tiny fibers that are able to get into microscopically tiny cracks and crevices of matte surfaces that allows Magic Erasers to clean such surfaces so well. Try cleaning your ping pong table with a Magic Eraser and bucket of water, and you'll see what I mean. Sponges, rags or scrub brushes simply don't have the microscopically fine fibers needed to clean dirt out of microscopically fine crevices a Magic Eraser has.
Alternatively, just cover your table with a blanket or even a linen sheet when not in use to keep it from getting dirty in the first place.
Hope this helps.