Well, at least as we think of terraforming. Mars could still be useful for housing people and colonies - it would just not be possible to put a breathable atmosphere on it.
Venus is a much better candidate for terraforming, although it would also have a similar problem with its magnetic field.
Realistically, however, once we have the ability to travel and construct reliably in space, it would be more efficient to construct facilities that meet our needs in the vastness of space than to try and repurpose planets. From a space faring standpoint, planets are something of a drag. Gravity wells make moving things into space extremely expensive and impractical while often having poor mineral quality due to the effects of the atmosphere and other compounds.