The large $x$ limit is only zero if $a+b>3/2$: $$_1{F}_2[{1}; {a, b}; -x^2/4]=$$ $$=-\pi^{-1/2}2^{a+b-2} \Gamma (a) \Gamma (b) x^{-a-b+3/2} \left[\cos \left(\tfrac{1}{2} \pi (a+b)-x\right)-\sin \left(\tfrac{1}{2} \pi (a+b)-x\right)\right]$$ $$+{\cal O}(1/x^2)$$
For example, when $a=b=1/2$ the function increases as $\sqrt x$, see plot:
When $a=b=3/4$ it does not decay at all:
