The reason that this is rather obvious is that there are two types of parties that we see in the twenty-nine year run: kiddie parties and parties for John and Elly. As we survey the scenes of cake-covered wall and frazzled Elly, it's clear that Lynn's take on birthday parties for children is something along the lines of "Whoop-de-freaking-do! You passed an arbitrary milestone so let's burden Mother with something elaborate that no one helps her with and no one seems to appreciate."
As for John and Elly's panicky over-reaction to the harmless apperance of pink flamingos on lawns, I should think that when they showed up for Elly's fiftieth, she all but shouted "YOU'RE CARVING MY TOMBSTONE!!!!" at people who just wanted to congratulate her on coming of age. This, I should think, is because most people were not raised with Lynn's old-fashioned viewpoint on the aging process: an all-too-brief flowering of beauty and sexual attractiveness followed by a rapid, irreversable descent into senescence.
Given that we're dealing with someone so into herself, she thinks people either all think like her or should be forced to do so, we can count on a very disappointing result: Over-the-top parodies of how nuts things can get in order to shame people into giving up something Lynn can't like. If she can't enjoy the process of planning parties for her children, if she didn't have one growing up because her own parents didn't see the point, if she thinks that people who want to wish her well are really mocking her for being too old to live, no one else should be allowed to enjoy them so she doesn't feel as if she's missing out on something.