How to improve LSTM accuracy on multiclass text classification?
50% is quite decent because you have five labels and random guessing model would have achieved only 20% accuracy. So you know your model is learning something.
The other thing you want to check out is whether this is suited to be a regression problem more than classification. For e.g, misclassifying a 5 (ground truth) into a 4 is better than misclassifying into a 2. Your current loss function treats both situations as equally bad. So, it's possible that your current accuracy measure is underestimating the models true performance.
You should also try out simple naive bayes model on top of bag of words. It will be interesting to compare how that compares with the more complex lstm.