Thread count is only marginally important in a mattress pad with 2 exceptions, after all you sleep on directly on the sheets, not the mattress pad. Exception one, higher thread count sometimes goes along with higher quality, the pad may last through more washings. Two, if you are trying to block dust mites and allergens you need a tight weave fabric with a high thread count.
I have nice 500TC sheets. I bought a 300 thread count $69 mattress pad, but wanted a spare. Then I bought a $29 200TC pad. I cannot tell the difference under my sheets.