It depends a little what you mean by “fun”, but the short answer would be yes, God is against fun.
God is not against pleasure, however, nor is He against rest and recreation.
It may not always be easy to tell the difference, but loosely speaking fun is typically exciting, addictive, and involves a fabricated (i.e. invented) form of entertainment with a strong mental component (e.g. sports, games, films). The reason God is against fun is that the exciting aspect disturbs mental clarity, and the addictive aspect engenders attachment to this world.
Pleasure is rather a side-effect of a natural act, e.g. eating or intercourse. Rest and recreation serve to recuperate from efforts. Both are fine and occasionally necessary.