I just tested it. You can pass a header by yourself but need to add

appendLocationNameToHeader: false,

Otherwise it will add the timezone again.

 
        {
            disabled: false,
            module: "weather",
            header: "YOURCITY",
            position: "top_right",
            config:
            {
                // See 'Configuration options' for more information.
                weatherProvider: 'openweathermap',
                appendLocationNameToHeader: false,
                weatherEndpoint: '/onecall',
                apiKey: 'API',
                apiVersion: '3.0',
                lat: '51.5',
                lon: '9.52',
                type: 'forecast',
                colored: true,
                fade: false,
                maxNumberOfDays: 3,
                degreeLabel: 'true',
                showWindDirectionAsArrow: 'true',
                decimalSymbol: ','
            }
        },