Dismissive term for college diploma

In Portuguese, a college diploma is colloquially referred to as a “canudo”, literally a “tube”. This usage is typically semi-dismissive, for example if you want to imply a diploma — the actual piece of paper — for a certain course is worth more than the education it represents.

Is there a similar word in English?

You'll sometimes hear "a piece of paper" used this way; see https://www.google.com/search?q=went+to+school+for+a+piece+of+paper for plenty of examples.

(But as choster notes: "The expression can be used to dismiss any kind of official document, however, not just diplomas or other educational credentials: marriage licenses, operating certificates, travel visas, building permits, arrest warrants, property deeds, constitutions.")

It's often said in English to be a 'sheepskin'. https://www.merriam-webster.com/dictionary/sheepskin

Because at one point, diplomas were printed on parchment made from sheepskins, the term “sheepskin” used to refer to them (first recorded in 1804): see the bottom of the page http://www.dictionary.com/browse/sheepskin