Why would topolects get described in terms of Mandarin? (声母变异、韵母变异)
Yes or No.

In Chinese linguistic study, there are three rules of distinguishing dialects from languages:


Linguistically, mutual intelligibility
Historically, derivation
Politically, anything except the standard


Dialects are picked out mainly because MSM gets politically recognized as a standard in China Mainland.

In practice, a minor reason is that they do not have matured or accurate writing system.