It depends on who you’re asking. Some people believe we should purposefully colonize other planets with life, in the hopes that someday some of it will thrive. For scientists, it is very bad, since it will make experiments that check for the remnants/presence/building blocks of life possibly return false positives. In the grand scheme of things, taking humans to mars is impossible without contamination of the red planet, and the scientific value of having a human on-site on mars likely outweighs the negatives of contamination.