It is because Ajax is asynchronous, the success or the error function will be called later, when the server answer the client. So, just move parts depending on the result into your success function like that :
jQuery.ajax({
type:"post",
dataType:"json",
url: myAjax.ajaxurl,
data: {action: 'submit_data', info: info},
success: function(data) {
successmessage = 'Data was succesfully captured';
$("label#successmessage").text(successmessage);
},
error: function(data) {
successmessage = 'Error';
$("label#successmessage").text(successmessage);
},
});
$(":input").val('');
return false;