To answer your question we must know what the signal source is. For example, if you are using the Portsdown without external video encoding, then the figures you report are to be expected. This is because the Portsdown's video to null packets ratio is set on the conservative side to avoid buffer underuns which could happen depending on the content being processed. It's a limitation of the Pi's computing power.
However, if you have a suitable Windows computer and you have one of the two supported SDRs then you can use the excellent F1EJP FreeStreamCoder or DATV Easy that leverage the video coding capabilities of the graphics chips. That can reduce the null packets to below 15%.
Ah, misunderstood what you were using as a source signal. I thought you were monitoring your own transmissions.
The portion of the packet stream that is filled with null packets is set by the encoder in the transmitter chain. In your case it is set by GB3PV's encoder, so there is nothing you can do about the sad face in Minitioune.
It's not in any way a reflection on your receive set-up.