I've heard this before where to keep your links "looking natural" you want to make sure your nofollowed to followed link ratio is a good and natural looking one. But just by saying a statement like that means your link ratio of followed and unfollowed links is not natural.
John Mueller from Google said "that's not true" when asked about this. The question was "It is ideally recommended to have a good distribution between the follow and the no follow links. This helps us to ensure that we do not get hit by any Google penalty related to links."
Here are those tweets:
That's not true.
— 🍌 John 🍌 (@JohnMu) July 31, 2020
It is true that these link tools do show a sites distribution between followed and nofollowed links. So it does make sense why some SEOs do talk about this. I do not think I ever looked at this metric before.
Forum discussion at Twitter.