Not sure how common superfluous quotation marks might be in English overall (I have to say I have not seen them that often in the UK, but sure can think of some examples in Australia, New Zealand and definitely lots in the US).
They’re absolutely everywhere in Mexico and in Spanish in general. This does make me wonder if it might be more of an Americas thing or just common in the Latin alphabet everywhere.
I’ll start with some peculiar signs I’ve found in Mexico, varying from very naïve art illustrations to some serious misspellings:
Cabrones by David Cabrera, on Flickr (quotation marks right from the get-go, this says: please no littering, “a**holes”)
Amor Eterno by David Cabrera, on Flickr (eternal love between man and his PPE in a former paper factory in San Rafael Tlalmanalco)
Perro de Agua by David Cabrera, on Flickr (look after the “water dog”, which I think might be the local term for river otters, in Santiago de Tuxtla)
Ortografía de los Cuetes by David Cabrera, on Flickr (“pure light” fireworks for sale in Valle de Bravo)
Forbidden the Step by David Cabrera, on Flickr (the Spanish is fine, but the English, well… the Rancho Nuevo Caves staff sure tried)
A Toda Madre by David Cabrera, on Flickr (“a toda madre” is an expression in Mexico similar to “freaking awesome”, so in this context “The Waterfall” is freaking awesome)
Se Rentan by David Cabrera, on Flickr (“cayaks” for rent at Las Orquídeas in the National Lakes of Montebello Park)
Baños Se Renta by David Cabrera, on Flickr (also for rent, toilets in Zipolite, really not sure how you rent toilets other than portable chemical ones)
Policía Feliz by David Cabrera, on Flickr (finally, a happy policeman in Villa de Mitla)