is there a way a non-admin with write access (role: Experimenter) can create readonly Personal Access Token, I want to connect GB to Claude using MCP server but want to use readonly API keys for obvious reasons.

Yes. PAT are available for all users. Whatever permissions they have in GrowthBook will extend to MCP, too. So, a call to create experiments, for example, will fail if the user doesn't have that permission. Currently, MCP will still show those tools as available, but the server will reject the call.