Microsoft Graph API returns mail message body as HTML

Don't you have to set the HTTP request header:

Prefer: outlook.body-content-type="text"

requestMessage.Headers.Add("Prefer", "outlook.body-content-type='text'");

As per the documentation https://docs.microsoft.com/en-us/previous-versions/office/office-365-api/api/version-2.0/mail-rest-operations

Edit:

View the documentation, this is the client class code: https://github.com/microsoftgraph/msgraph-sdk-dotnet/blob/dev/src/Microsoft.Graph/Requests/Generated/GraphServiceClient.cs

Here is an example from the link you're following:

private static GraphServiceClient GetClient(string accessToken, IHttpProvider provider = null)
{
        var delegateAuthProvider = new DelegateAuthenticationProvider((requestMessage) =>
        {
            requestMessage.Headers.Authorization = new AuthenticationHeaderValue("bearer", accessToken);

            return Task.FromResult(0);
        });

        var graphClient = new GraphServiceClient(delegateAuthProvider, provider ?? HttpProvider);

        return graphClient;
 }