hello
the standard SSR's are designed for AC - they will turn off at each zero crossing.
if you use it for DC it might trigger once and then remained turned on until you remove the power.
might trigger: 24V might be too low for the triac inside, and for it's driver chip (if it has one), and usually the driver looks for zero crossing to actually turn on the triac.
if the SSR you have is specified for DC, then it should be fine as long as you're within it's operating specs.
get one that's at least 20-30% over your heating pad current - it might have higher current draw when cold.
even more if it's an ebay / aliexpress / amazon "special"
you can make your own from a mosfet and some other parts, but the premade ones have the advantage of a case / heatsink + you get it working right away