Most European countries have proportional elections, at least for some levels of the government. Such elections give a pretty good idea of the popular support for various ideologies. On the average, parties that are broadly aligned with US Republicans get ~25% of the votes, though there is a lot of variation from country to country.
>Share of respondents who said Trump would be good or bad for their country..
No no, that is not what is being said here. To match with it, the poll should ask if they would like someone like Trump to be their leader, not if Trump in US would be bad for their country.
How is this measured actually? By what the media and public institutions say?
Is there poll targeted at the common working man that says the same? I highly doubts so.