Who is "the blessed virgin of Yorba Linda"?

I am fairly sure that this is just a silly joke about the fact that "Fez is foreign" ("FEZ" is a abbreviation of "Foreign Exchange Student"). Unfortunately, we never find out where is he from and there are quite a few jokes about it (quoting from memory):

[Fez] I might get back to Brazil... And then take a plane back home.

So what is the name of your home country?
[Fez] Depends who do you ask - Brits or Dutch [...]. Brits hate our guts, so they won't say it. And we can't understand the Dutch.

In the early seasons we find that Fez is quite religious (or at least he was - he used to live in quite Christian family that was treating Rock music as a spawn of Satan), so it is not a stretch of imagination to say that he might have a tattoo of a "Blessed virgin from [foreign named place]". "Yorba Linda" sounds foreign enough to an average American and of course it will be a double joke if you know that it is not so foreign after all.

The only Blessed Virgin is Mary, and she has many, many appellations. In particular a lot of places have taken her as the patron saint of their region, and use "Blessed Virgin of ..." or "Our Lady of ..." as their local reference of her. Typically these places are European and old, and their names carry a certain romance or mystique to the (not extensively traveled) North American ear. Substituting a local place name, especially one with connotations of work-a-day suburbanism, brings a humourous element. (Though Wikipedia tells us that some US cities do have patron saints, so it's funny but not absurd. And to a Wisconsinite of the '70s, Yorba Linda might have been as mysterious as Lourdes, so the joke was for the benefit of modern west-coast audiences.)

A very similar joke which crops up from time to time is a play on the habit of famous brand names to associate themselves with major world cultural cities: "ACME CLOTHIERS - London - Paris - Milan". T-shirts printed with the local backwater place name added to the list are a staple at county fairs.

It also marks Fez as Catholic. Catholic vs Protestant has been a pretty big part of North American identity; JFK's Catholicism was of concern for enough voters that he addressed it in speeches. And the focus on saints, veneration and patronage is particularly opaque to many North American protestants and an occasional source of humour, xenophobic but not-quite-racist-enough to be out of bounds for a sitcom. That a foreign student might have a saint's tattoo would fit into the same category as having several sisters all named Maria.