In some signatures, I've seen an image with text:

(username) has (number) internets
To give an internet, click this button

Is it okay for this site to be linked on Scratch? And what should I do if I see one of these signatures?

As far as i'm concerned, internetometer is completely safe. I've used it on other forum sites but not scratch - ultimately this is a question for the team.

Yes

it should be appropriate: its just a button you can press