It’s applicable if one hypothesis is simpler than the other. Panspermia simply defers the origin of life elsewhere and now you have to explain how seeds survived travelling through the hostile environment of space and impacting on Earth, which complicates it quite a bit.

Ockham's razor is either useless, or a special case of Bayes' Rule. People argue endlessly about it because they fail to recognize they are assuming different priors.

If the Urey experiment is reasonably representative of conditions on primordial Earth, no further work is required to get amino acids. So for some people P(terrestrial primordial amino acids) is ~1

If the Murchison meteor and other evidence is correct, amino acids are common in the universe. Data from the Philae lander may tell us more about this. So for some people P(extra-terrestrial primordial amino acids) is ~1

Nor are these exclusive groups. It appears amino acids aren't all that hard to make, so they may have both fallen from the sky and been created on Earth.

But... amino acids are not life. There is a further step that is required for the actual pan-spermia hypothesis: the synthesis of more complex molecules, like RNA and DNA. If you think P(RNA|amino acids) ~ 1 then pan-spermia becomes at best irrelevant: not impossible, but not very interesting.

If you think P(R/DNA|amino acids) ~ ε then pan-spermia becomes very important, because it is far more likely than not that life on Earth is due to RNA or DNA that descend from an original synthesis that happened elsewhere.

Evidence suggests RNA synthesis given amino acids is not very probable. I don't believe it has ever been observed to happen spontaneously the way amino acids are spontaneously synthesized in the Urey experiment. So pan-spermia cannot be dismissed on the basis that it is uninteresting.

DNA propogating itself across the universe is also not very probable, so the two hypotheses remain in competition, and it isn't obvious which is more plausible. Pan-spermia requires things like "DNA can survive re-entry." So the OP is correct: discovering that DNA can survive re-entry makes "Life originated elsewhere" more plausible.

You'll notice I never used the word "simplicity" in any of this. There is no simplicity: only probability and plausibility. Evidence is more or less probable. Propositions are more or less plausible. Bayes' rule is the only way to update our beliefs consistently, such that no matter what order we get the evidence we come to the conclusion.