This sentence is from Doraemon: そんなのめずらしくもないよ. I think it means "It is not that rare/uncommon". But I don't understand the use of く も here. Should it be interpreted as めずらしく + も? 1. Why they use めずらしく here? I thought i-adjective + く = adverb while it should be noun in this case. 2. Why they use も instead of は? Thanks
Something like that is not really rare.
It's めずらしく + も + ない. めずらしく is the ku-form (continuative-form) of めずらしい.
This も is like "well", "(not) quite", or "(not) really". This type of も appears in many sentences, and it essentially makes the sentence sound milder and more reserved. For example, そうではない ("That's not correct") can sound harsh, but そうでもない ("That's not quite correct") sounds milder.