Coming back to this: my definition of "serious person" is more or less someone who thinks life is serious and should be taken seriously. Someone who doesn't shy away from deeper or more serious issues, who doesn't think it's just about pursuing the maximum of fun and glitter. So I suppose I'd describe it as a "good thing" overall.
I certainly do agree that constant serious frownyness can be pretty dire. Someone who can never lighten up or laugh about things can be pretty sad to be around. I have a pretty intense silly switch. Sometimes I just enjoy myself and skip about.

I like it when I make others laugh, too. But I think I'm considerably more serious at the core. I certainly do have an easier time turning on the silly switch than I did when I was younger, ie. in my teens. I think I must have been unendurably serious a lot of the time then.
Conversely, people who turn everything into a big fat joke really don't go down well with me. You start to realise that you can't say anything to them, even something deeply serious, without them laughing about it - basically laughing at you. I'm more likely to start keeping my distance from such people or not saying anything much around them.
So, as with everything - I think there's a good balance to be found.