One technique doesn't suddenly make her the strongest. Sakura was a freshly minted chunin at the start of Shippuden and hasn't shown any visible progress till that asspull. I'm sure she has improved through general osmosis but let's not kid ourselves, she was nowhere near Kage level before that.
Also Sakura didn't (unfortunately) have to recover from being cleaved in half so she didn't exactly accomplish Tsunade's feats.
It's an asspull because she pulled it out of proverbial magician's hat. It was never hinted or mentioned that she was working on that technique. Kishi just thought, "Oh shucks I'm going for a Team 7 centric ending so I probably should give Sakura some upgrade too."
Also Mei > Tsunade