i don't think tier is her problem just the fact that its the skilled players whom play her. theirs always gonna be a character with average to no tier ranking that can best top tier simply because the players are more adept with that character.
Pai's Tier Ranking can be really difficult to determine because of how different her gameplay focus is from everyone else (such as Alpha-152 to an extent in a sense). She can be incredibly aggravating to deal with online because of how many buttons she can keep pressing continuously, but offline she seems extremely solid too. I'm reckoning either a high "B" or a low "A".
She is quite good, but it mostly seems like she's doing 1 thing REALLY well. She has the same weaknesses as the other VF characters and is the most vulnerable to good spacing. If Pai isn't able to maintain momentum, she gets hurt.