Oh dear, where to begin.
What you are forgetting is that as the #1 tennis player in the world and arguably the best player to ever live, when she plays at her top level she is as close to unbeatable as anyone has ever been. What she says here is what every single player in the history of the game has ever said when they lose -- that they should've played better. That's not a problem.
Here is Djokovic after his shock defeat to Vesely this year: "There are a very few things I could take out from today's match as a positive - I was playing really, really bad." - https://www.clubcall.com/tennis/i-wasnt-good-enough---djokovic-1806729.html
Murray after losing to Djokovic: "I could have played better, like I said. I didn't hit my forehand as well as I could have done. I had 25 or 26 more unforced errors than him in the end." http://www.asapsports.com/show_interview.php?id=116391
Rafa after a subpar clay season last year: "I played bad in Barcelona. In Madrid, I played one bad match, the final. I have not been consistent enough for the first part of the season. I am not as good as I was." https://www.theguardian.com/sport/2015/may/12/rafael-nadal-i-am-not-as-good-as-i-was-novak-djokovic-rome-masters
What you are also conveniently ignoring is the fact that those quotes are just that. Quotes from 10 minutes long press conferences and multiple interviews where the players I've quoted and Serena included ALWAYS praise their opponent for their win.
After losing to Kerber: "I was actually really happy for her. She's been around a really long time. She played so well today. She had an attitude that I think a lot of people can learn from: just to always stay positive and to never give up. I was really inspired by that. So, honestly, she's a really good girl. If I couldn't win, I'm happy she did. http://www.asapsports.com/show_interview.php?id=116220
After Muguruza when asked if her adductor injury caused the loss: "I'm not one to ever make excuses and say, like, Oh, my adductor was hurting or whatever. I think at the end of the day I didn't play the game I needed to play to win and she did. Adductor or not, she played to win. You know, that's what she did. She has a bright future, obviously. She knows how to play on the big stage and she clearly knows how to win Grand Slams." http://www.asapsports.com/show_interview.php?id=120013
Again, I'm still waiting for some evidence of those disgraceful things she does ON COURT, too.
Oh, and you should probably read this - http://www.eurosport.co.uk/tennis/french-open/2016/true-champion-serena-williams-deserves-every-accolade_sto5628693/story.shtml