Can we validate API response with app result?

Add OkHttp jar or anything similar which will do GET, POST etc.
Sample is given here http://square.github.io/okhttp/20.
Make a generic code for that when you are running appium code import that code to the main code pass API URL and headers and get response same like your app, store it in JSON array use JsonPath(https://github.com/json-path/JsonPath8) get the data what you need and compare both.

Don’t have any reference links if you want you can refer below sample which i roughly did
Below code is for GET method

function will accept URL and headers and return response

String[] get(String url, Map<String,String> headers) throws IOException {
client = new OkHttpClient.Builder().connectTimeout(120, TimeUnit.SECONDS)
.writeTimeout(120, TimeUnit.SECONDS).readTimeout(120, TimeUnit.SECONDS)
.followRedirects(true).followSslRedirects(true).
build();
response_received= new String[3];
Request.Builder builder = new Request.Builder()
.url(url).header(“User-Agent”, “Mozilla/5.0 (Linux; Android 4.0.4; Galaxy Nexus Build/IMM76B) AppleWebKit/535.19 (KHTML, like Gecko) Chrome/18.0.1025.133 Mobile Safari/535.19”);
headers.forEach((key,value)->builder.addHeader(key, value));
Request request = builder.build();
System.out.println(ExecutionLogGenerator.Logger(classname)+"–> HTTP GET “+ url);
try (Response response = client.newCall(request).execute()){
System.out.println(ExecutionLogGenerator.Logger(classname)+”<-- HTTP “+response.code()+” “+url+” [Success : “+ response.isSuccessful()+”]"+" [Message :"+ response.message()+"]");
response_received[0]= String.valueOf(response.code());
response_received[1]= response.message();
response_received[2]= response.body().string();
return response_received;
}
}

but comparison its based on requirement you can go through the samples given here
https://github.com/json-path/JsonPath9 you will find something suitable for your requirement.

you’re welcome