I'm not sure that is a good measure

Imagine I crated a huge huge number of nodes that do nothing. The number is so huge that my nodes that do nothing are 99% of nodes. So 99% of nodes do nothing and 1% of nodes handle all payments. 

Has the network centralized? By this metric yes by a lot.

https://stacker.news/items/20470

"When gossip messages were first being collected April 2019, it was found that 10% of nodes accounted for 70% of betweenness. Since then, centrality increased to the point that the 10% nodes in the network account for 90% of centrality. That means, assuming payments occurred at random. that this 10% of nodes handles 90% of network payment traffic. This can adversely affect the privacy of the network."

See also: the Gini coefficient

random_

. In addition to the LN’s long known application-level centralisation,
recent work has highlighted its centralisation at the network-level which makes it vulnerable to
attacks on privacy by malicious actors.

what do they mean centralization on network level?

lightning

Payment Censorship in the Lightning Network Despite Encrypted Communication

Rsync25

> . In addition to the LN’s long known application-level centralisation,
recent work has highlighted its centralisation at the network-level which makes it vulnerable to
attacks on privacy by malicious actors.

what do they mean centralization on network level? 