Hi @tanasis
It's up to you to decide what are the optimal values for your server for certain services. These thresholds (and the alerts) are set just for monitoring and notification reasons and are adjustable per your needs. According to your htop output, there should be enough memory though so it looks safe to increase a threshold a bit and keep an eye on how it goes.
In general, if you do not have experience in server administration, we recommend that you keep the default Monitoring configuration. The preconfigured thresholds and enabled notifications are optimal. Otherwise, you can set your own thresholds and turn on and off notifications for particular resources.