Not really. It is serious enough as a theory which tries to unify quantum mechanics and Einstein's gravity. Quantum theories dominate the micro world at the atomic scales and less, where as the gravitational interactions describe the universe at large scales; motions of heavenly bodies such as planets, stars and galaxies.

But then, you may ask, why do I need to apply gravitational force at the atomic and sub-atomic scales where we know that sub-atomic and nuclear forces play dominant roles? Strong, weak and electromagnetic forces are much stronger than gravity at sub-atomic distance scales, and we can safely neglect gravity in comparison with them. Then why search for a quantum level description of gravity? The answer is the following:

(A) In a big bang scenario the universe began with a singularity. At initial stages, thus, the quantum version of gravity was important. These quantum effects have later formed into the very seeds of further developments of large scale structures which we see; stars, galaxies, clusters of galaxies, voids, filaments and so on. (B) We know (and it is confirmed) that there exits black holes in nearby regions of the universe. Physics near the black hole singularity must be understood by quantum gravity where again classical gravity offers an inadequate description.

In summary even though string theory is at the moment a purely mathematical construct, and it is yet to be confirmed by experiments, we must continue our studies in string theory to understand mainly two things (1) Physics of the primordial stages of the universe (2) Physics of black holes.