I have always been of the opinion that, if a word is familiar to an adult native speaker of English, it should be classed as common.
I also feel that if a short adjective in -y is classed as common, then the comparative and superlative forms of that adjective, made by adding -ier and iest to the root word, should also be common, on the grounds that they too would be familiar to an educated native speaker:
eg trendy, trendier and trendiest should all be common, because the first form trendy is common.
I would go further and say that a noun formed by adding the suffix -ness to a common adjective should also be common - trendiness should be common because trendy is common.
So, if nerdy is common, then nerdiness should also be common.
Should nerdy be common? That's another question.
Just my opinion.