I think this Google translation of a news article is quite good: LG München: Einbindung von Google Fonts ohne Einwilligung (look at 5., 6. and 7.)
As @roman_r_m already said a German website used Google Fonts and has to pay 100€ in compensation to a user. Its important to note that there was no opt-in option. Google Fonts where loaded automatically and therefore the IP address was transmitted automatically. GDPR allows providers to claim a "legitimate interest" to transmit certain data, but it is easy to use local fonts on a web server. So there is not legitimate interest to use Google Fonts. Therefore the company has to pay compensation to the user.
Its the first instance so an appeal to a higher court is likely. Moreover it is just one district court. Only time and more lawsuits will tell if using Google Fonts is a GDPR violation. The court decision deals only with Google Fonts, not with YouTube embeds, Adobe Fonts, Hunspell or any other form of external services. So this decision does not have a direct impact on Joplin.
But it shows where we are headed in Germany/EU (and other regions that introduced similar legislation) IP addresses are personal data, therefore protected by the GDPR. So "simple" GET request are a GDPR violation, certain exceptions might apply.
I personally think you cannot compare Google Fonts and Googles Electron CDN, because you cannot add every dictionary to a Joplin release. But perhaps users have to opt-in before Joplin makes a connection to a Google server could be required.
(I am not a lawyer, so I am just guessing based on my experience with german courts in the past.)