Colonizing the bottom of the ocean would be orders of magnitude easier. Or the South Pole. Or Kīlauea’s open lava pit.

While it is true that you have different sets of problems to sole, nope, they are only cheaper to get to, not necessary easier to colonize, except maybe the South Pole where you just need to build something that only need to withstand the cold, which is easy enough and you could go outside without a space suite or something similar.

The problem with colonizing Mars is the cost, which have as a consequence the cost of everything you send to that place.