Either get into counseling and find out what's behind her issue with lightheartedness and humor, or let her be a sourpuss if she wants, but everyone else should feel free to joke and laugh (not at her expense) as they please. No one has to allow her to "squash" their laughter. She's free not to laugh, and everyone else is free to chuckle, cackle, or guffaw as they please.
What's probably a good idea is both. Don't allow her to detract from anyone's expression of joy/fun, but do encourage her to see someone. You say "for years now" she's been been that way, which implies it wasn't always the case. What changed? What caused her to go from not being "so uptight and selective" about what she finds amusing. If you don't want to talk to her about it, then at least state to her that while she has the right not to find things funny and not to laugh, but other people choose to and there's nothing wrong with that. In fact, it should be encouraged. People who laugh and are lighthearted tend to live longer and have better quality of life than people who are uptight and stressed all the time.