Is "less than eager" a native English idiom?

I'm sure I've read the "less than eager" expression a couple of times, but without being a native English speaker I can't tell for sure if that's an English idiom or a literal translation of an idiom from another language.

A quick search for "less than eager""usage" returned multiple translations engines offers to translate it to various languages as top results. It's never a good sign.

So, is this a less known native English idiom? If not, is there a more appropriate one which I could use?

It's idiomatic, but it's not an idiom—a phrase or usage whose meaning is not determined by the meanings of its parts. You can use less than readily with just about any adjective which denotes a measurable quality:

less than eager
less than perfect
less than unanimous

As the Comments suggest, less than eager is ordinarily used in an ironically understated sense more or less equivalent to strongly averse.

I wouldn't describe it as an idiom, but it's certainly idiomatic. It's used with other adjectives too. The relevant definition of less is:

(less than) far from; certainly not:
Mitch looked less than happy

I can't tell you why a regular Google search behaves like that, but I had more luck finding examples of usage with a Google Books search.