What you said was, "pursues what it wants by lying."
This means you associate Pisces with lying. Tell that to all of your Pisces friends, and see how many you retain. This is just bad astrology.
Again, Mercury rules liars. If you found something like a Mercury square Pisces Mars, that might combine "pursuit" with "lying." But Mars on its own, no.
If we use key words, a planet in Pisces tends to be emotionally adaptable. (water, mutable.) A Pisces Mars, consequently, is less likely to "pursue what it wants" (a so-so definition of Mars) than most other signs.
I think you're conflating Pisces, Neptune, and the 12th house. Neptune does not deal in reality, but it is not thereby necessarily deceptive. The 12th house traditionally includes secret enemies and misfortunes of various sorts, so there is kind of a connection with lying there. But a sign is not a house is not a planet.