If the API Gateway expecting format and your Lambda function’s response does not match, you will receive a Malformed Lambda proxy response.
{
"isBase64Encoded": true|false,
"statusCode": httpStatusCode,
"headers": { "headerName": "headerValue", ... },
"body": "..."
}
If you do not want to use Lambda proxy integration, then just log on to API Gateway console and uncheck the box which says the same.