Trusting Cognito accessToken in lambda (authorizer in use) - Authorization

Is it safe for my lambda function to trust the accessToken passed by the user and checked by the lambda authorizer to perform CRUD db operations?

For example:

const authToken = event.headers['Authorization'];
if (!authToken) throw new Error('No auth token found so no username');
var decodedToken = jwt_decode(authToken);
const userName = decodedToken.username;    //---- BUT CAN WE TRUST THIS? ----
let params = {
  TableName: "myTable",
  IndexName: 'userName-gsi',
  KeyConditionExpression: 'userName = :userName',
  ExpressionAttributeValues: {
    ':userName': userName,
  },
  Limit: 1,
};
let data = await dynamodb.query(params).promise();
return {
  statusCode: 200,
  headers: utils.getResponseHeaderApplicantifyCors(),
  body: JSON.stringify(data.Items[0]),
};