Never said she wasn't strong. She isn't top tier. She ain't 1v1 any admiral she ain't beating any yonko.
At best she would be yc1 level. Her ability is just convenient. However it won't work on top tier haki or other top tier fruits
It depends on if she has Adv CoC. To me, any REAL top tier has it. That's why, even though I love Law so much, I can't put him near the top 10 yet.
If Boa has Adv CoC like I think, she's top tier, around the Yamato tier. Capable of clashing with Kaido, but not beating him.
If she doesn't, then you're right, she's below top tier. You're underestimating her fruit, though: she was able to turn Pacifistas into stone.