“OF COURSE YOU AREN’T!”
And that’s all there is to it, isn’t there? Usagi is shocked Chibs could still think she’s nuisance to Usagi after the HUGE “EVERYONE LOVES YOU DEARLY” musical number Usagi gave.
But that might be part of it really, since Black Lady causes a lot of pain for everyone Chibs may wonder if that made her even more of a problem.
But Usagi doesn’t care about that. Usagi understands.
And Usagi doesn’t think about Chibs as a nuisance now, perhaps she never did deep down (I mentioned before that her own deeply buried affection for Chibs may have led her to be even more fearful everyone would love Chibs more). Chibs bugs the shit out of her sometimes, sure, but that doesn’t mean she doesn’t enjoy her being around at the same time. (See: Rei). She wouldn’t have it any other way. In its own way, even their bickering has become affectionate and fun.
Finally, Chibiusa gets what she truly needed: the assurance she needs that Usagi doesn’t consider her a burden, that Usagi wants her around, that she’s important. Usagi tells her what she needs to hear. as loudly and clearly as she can. No miscommunication now.. Only love.