Is it possible that tablet can be a mobile monitor? So I can connect my Mac mini to it through HDMI port.
HDMI ports are not bidirectional when sending audio/video. The HDMI port on your tablet is output only, so that you can connect it to a TV or monitor. You will not be able to use it as a second monitor in that way. However, there are alternatives that would allow you to do so, depending on the tablet's OS and hardware. That is a whole different animal, however.