Sure, but how each person defines fun will vary.
That goes without saying.
You'd think so, but often enough it doesn't come across that way. Often enough when someone says they watch something "just for fun" the implication is you shouldn't think too much about watch you're watching.
So I can say I enjoy TOS because of the fact that it's not only visually interesting but it also exhibits a good measure of intelligence in how it's put together. From that my sense of "fun" is going to have certain expectations. If I have to suspend any measure of critical thinking too far then I'm not having fun anymore.
On the face of it a show like Futurama
looks like utter nonsense and yet it's often pretty damned clever in how it's put together.