You are right to a point - it can be a security issue. This is one case where it's a trade between convenience and security: you must decide what is the right balance for your particular use case.
Jelastic gives you access to the Apache config on the MySQL node (so you can configure HTTP auth, or IP based restrictions - or even disable phpMyAdmin entirely if you don't need it). You also have the ability to customise the firewall rules (e.g. to restrict phpMyAdmin access to your own office IP address).
One note about the firewall rules - unless you add public IP to your MySQL node (not recommended), you will be accessing via the Jelastic resolver (shared proxy) so you cannot selectively block this way (the IP that connects to phpMyAdmin is the proxy, not your office IP).
I recommend to consult with your provider's support team to get exact assistance for your particular needs.