Is a - Att Kagero okay?

her attack is her second most important stat.

I don't recommend upgrading a 4* without perfect IV. Plus, Kagero -att is just not worth it. Especially in the new Arena maps. Even +Att Kagero doesn't do well on those maps with defense area.

The only reason why you might still want to upgrade Kagero, is so that she can pass on her dagger to someone else. While there aren't very good units yet that do her job better, if you have a +atk Gaius lying around; with +atk he has the atk stat of -atk Kagero, but he has a much higher speed stat, meaning he can double fast infantry more often than Kagero.

If he happens to be -HP +Atk it's even better, you can add Ardent Sacrifice to get below 75% and make him inherit Desperation so his doubles will follow up.