Interesting question. On the one hand, we read that "men are that they might have joy" (2 Ne 2:25) and "In the world ye shall have tribulation: but be of good cheer; I have overcome the world." (John 16:33) On the other hand, we read "cease from all your light speeches, from all laughter, from all your lustful desires, from all your pride and light-mindedness, and from all your wicked doings." (D&C 88:121)
In my mind, it's important to pay attention to the context of laughter. Laughing scornfully or mockingly is wrong. (See Gen. 17:17, 18:12-13, and Luke 8:53.) We're also advised to be "sober" (1 Pet. 5:8), which can mean both "serious" and "not drunk." (If you've ever been around drunk people, it's not hard to see why inebriation and silly laughter are associated.) These both make sense in a larger context: It's wrong to be scornful or mocking, whether or not you laugh, and drunkenness, which robs us of our ability to make good judgments, is also to be avoided.
We might draw from all of this that it's better to be serious than sorry, or that more serious = more righteous. I know some people who certainly seem to believe this is true, but I can't accept it in my own life. I am someone who definitely likes to laugh. I think the world is a funny place, and I find great amusement in everyday life. So it's a little distressing to be told that I can't laugh loudly (or at all). More importantly, I feel that my ability to laugh about my problems gives me a sense of peace and hope that I couldn't find by other means.
Even President Hinckley has been known to make the congregation laugh in General Conference (and I probably laughed loudly, for such is my laugh). Better yet, Job contains a reference to God
making us laugh: "Behold, God will not cast away a perfect man, neither will he help the evil doers: Till he fill thy mouth with laughing, and thy lips with rejoicing." (Job 8:20-21)
I think the Preacher put it best: "To every thing there is a season, and a time to every purpose under the heaven: . . . A time to weep, and a time to laugh; a time to mourn, and a time to dance." (Eccl. 3:1,4)
The LDS Guide to the Scriptures
explains that being light-minded means "treat[ing] lightly sacred things." That seems qualitatively different from daydreaming, to me. Being disrespectful of sacred things is wrong, but that doesn't mean that there couldn't be a "time to daydream, and a time to focus." (Of course, you could spend the rest of your life trying to figure out exactly when
you should laugh or weep or daydream . . . but I don't agree that every second spent wishing or laughing is going to "count against you.")
Worrying is rarely productive; just do your best and live a joyful life.