From various sources such as Wikipedia, NASA, and various published papers, an orbital resonance is:
when orbiting bodies exert regular, periodic gravitational influence on each other, usually because their orbital periods are related by a ratio of small integers
So a ratio of $1:\sqrt{2}$ wouldn't count.
What's more it wouldn't even make sense to talk about a ratio of $1:\sqrt{2}$. That requires that the resonance can be defined with infinite precision and that just isn't possible. Orbiting bodies have so many perturbations on them that if you look with enough detail, the resonance is never perfectly $1:\sqrt{2}$ or 1:2 or 2:3 or whatever it may be. You'll never achieve a $1:\sqrt{2}$ resonance.
On a related point, Connor Garcia points out in a comment that the rational numbers are dense in the reals, meaning that you can substitute any rational number with arbitrary precision that is arbitrarily close to $\sqrt{2}$. For example, you might use 1.414213562373095 instead, but then your ratio becomes 1:1.414213562373095 or rather 1000000000000000:1414213562373095 which is a completely meaningless resonance as it will have no significant gravitational effects. Such nonsense resonances result in relative positions of the two bodies which is essentially random and thus not really a resonance.