Is taking ideas from an art critic plagiarism?
Yes, it's plagiarism if she learned novel or interesting ideas from an art critic and then used them in her own paper without crediting the source.  It doesn't matter whether the ideas are facts or opinions.

For commonplace opinions with nothing novel or interesting about them, there's no need for a citation.  However, that's presumably not the case here, since one couldn't attribute such opinions to a specific critic.