Yes. It says スッキリ, though from this perspective it's upside down.
......
Dang, I thought it was some version of the artist name (much like the Twitter username that frequently gets added), so I've been tagging illustrations with those with the artist_name + signature tags.
C-Cirno-chan... yes, you're right.
It is indeed refreshing.
But it's a little bit embarrass-Cirno:
Dai-chan, with just those strings removed, it must not be all that refreshing?