It is not. The replacement will be selected by a delegates to a convention, 500 selected by county-level Democratic committees and 100 for the state committee members. So a fully insider selected faux-electorate with bonus superdelegates just in case.
https://politicalwire.com/2026/07/08/maine-democrats-ready-convention-if-platner-exits/
A poll would matter if the people at the convention were voters. They're not. They'll be selected by the county committees. Mills might be a step too far, but they'll sub in a bland moderate.