You're being very inconsistant.
Firts you say its average.
Then, assuming there's 37 playable characters, your numbers place her above ~60% of the cast, which is slightly above average assuming the characters are split evenly between ones with high and low strength.
Then you say her attack stat is outright bad.
Which is it?
All of the above.
First off, I didn't say EXACTLY average, there is no 'exact' with the way the distribution is (Well, I don't think so, but my guess is that it doesn't average out to a single flat number). The distribution is actually from a minimum of 2, up to a maximum of 22, but it's not linear at all. (Second-best is 17, best is 22, just to put it in perspective, and who holds these two values should be obvious)
EDIT: On a side note, I think the actual average value, calculated out, would probably be closest to a growth rate of 9... which goes solely to
Second, she TIES with three characters as well. So that's 40. Even still, if she's at '60%'.... that's still within 10% of the 'standard' value. That's not exceptional either way.
And third, just because her stat growth is actually average for ATK
across the entire team... for characters
who require ATK exclusively for their damage formulas, Nitori is terrible. Of the characters who only use ATK, only Chen places lower than Nitori in terms of ATK growth (I may be missing one of the far later characters however). A few characters who have composite spells, such as Cirno, also have lower ATK when they use the stat, but they've also got MAG as well.
This happens a lot. A lot of characters are 'average' in a lot of stats... but to be frank that doesn't necessarily mean they're good at them at all. This is especially the case for ATK and MAG, because a lot of characters rely at least somewhat more on one than the other - that Nitori's reliant stat matches up to a character who barely uses it
should say something. And unlike the girl she beats - Chen - she can't buff her own ATK and has far longer delays, too.
It really is the fact that her damage multiplier on Megawatt is one of the game's best, that actually makes her notable as a damage-dealer.