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The books are okay but nothing that special. If those are your favorites try reading more
I agree that it's not a happy ending. It's bittersweet. 
Losing her sister was obviously tragic. As for killing Snow, her failure to kill him isn't sad. She knew the guy was going to die soon anyway, so there was no need for her to take his life. His life was already ending. Since there was no need for her to kill him, she chose to kill the next would-be dictator. 
The capital falling was "happy" in the sense that it was a positive thing, but revolutions are horrible events to live through even when they're righteous and successful. The happiness caused by (some) revolutions is rarely experienced by the revolutionaries themselves, who will always have any possible happiness tempered by their trauma. It's a gift revolutionaries give to future generations.
Katniss's children are the fortunate recipients of that gift; in them, she's able to see the joyful results of Panem's fall. >

It’s the smile at the end of the movie I swear changes the entire meaning. In the books it’s clear she’s beyond traumatized from everything and will never truly recover. >

Goals change with the person >

I think it is bc she found love and has kids that won't be sacrificed and is happy. Obviously it would be better ending if her sis didn't die, still mad about that, but I guess not everything winds up perfect >

Thats the entire point. >

her main goal was to kill snow because he is the leader of panem but no she didn’t kill him or save her sister but i think the ending is still a happy one. she never wanted kids because of the world she lived in was terrible so showing her with a family at the end with two kids means they must live in a better world now. therefore imo it’s still a happy ending but i see the point you are making