County Councillor is a paid political appointment, in other words, it's a job. But it's a job unlike any other, as the employer is the general public. It's human nature for these folks to trumpet their achievements in order to win another four years' employment. If your Parish Council doesn't want to see its newsletter being use for political campaigning, it should say so. There are no rules or guidelines governing this.
All of my parishes have strong relationships with our district and county councillors, including attendance at meetings, which enables us to work with them on issues more relevant to them than to the parish council. I have one district councillor on my own parish council and she is a hard working member of the team. Individually, most of us support her election campaigns, as we can see the difference she makes, but our public communications don't single her out from the rest of us. Our website includes details of each of our district councillors, our county councillor, our MP, our PCC, our elected mayor etc etc.