要在ASP.NET MVC中返回小写属性名称，可以使用自定义的JsonResult类来实现。下面是一个示例代码：

首先，创建一个自定义的JsonResult类，继承自JsonResult类：

using Newtonsoft.Json;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;

namespace YourNamespace
{
    public class LowercaseJsonResult : JsonResult
    {
        public override void ExecuteResult(ControllerContext context)
        {
            if (context == null)
                throw new ArgumentNullException("context");

            var response = context.HttpContext.Response;

            response.ContentType = !string.IsNullOrEmpty(ContentType)
                ? ContentType
                : "application/json";

            if (ContentEncoding != null)
                response.ContentEncoding = ContentEncoding;

            if (Data != null)
            {
                var jsonSerializerSettings = new JsonSerializerSettings
                {
                    ContractResolver = new LowercaseContractResolver()
                };

                var serializedObject = JsonConvert.SerializeObject(Data, jsonSerializerSettings);
                response.Write(serializedObject);
            }
        }
    }
}

然后，创建一个自定义的ContractResolver类，用于将属性名称转换为小写：

using Newtonsoft.Json.Serialization;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;

namespace YourNamespace
{
    public class LowercaseContractResolver : DefaultContractResolver
    {
        protected override string ResolvePropertyName(string propertyName)
        {
            return propertyName.ToLower();
        }
    }
}

最后，在控制器中使用自定义的LowercaseJsonResult类来返回JSON数据：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;

namespace YourNamespace.Controllers
{
    public class YourController : Controller
    {
        public ActionResult YourAction()
        {
            var data = new { Property1 = "Value1", Property2 = "Value2" };

            return new LowercaseJsonResult { Data = data };
        }
    }
}

这样，返回的JSON数据中的属性名称将会被转换为小写。