It probably did have at least some effect, as was mentioned by several participants in the event, but the margins it would effect don’t matter at all because of the absurd amount that Beamon cleared the record by.

Sure, thinner air and weaker gravity due to higher elevation. But those alone don't come close to accounting for the record, compared to the previous record. The previous record holder could've done his jump from Everest altitude and, mountain slope notwithstanding, not beaten Beamon.