Declined
Here is the graph of z = cos(x y) in GeoGebra. The output is quite rough even the option "Quality" selected
The graph of the same function generated with the defaults options in Mathematica 9:
As bad as GeoGebra output. But, Mathematica has an option to improve the quality of the graph:
Yes, you have sacrifice performance, but you get a much better image, suitable for use in Calculus textbooks, for instance.
My suggestion: implement em GeoGebra such option to increase the number of points used to improve the quality of the graph.
I like this idea 
Sorry, it's not as easy as it sounds to do that.
What you can do in the meantime is to split the surface up, for example https://www.geogebra.org/cl...
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