Terraforming Mars doesn't require Nuclear fusion per say, just energy, in fact using your average coal or oil energy station would much much more effective (as it produce, greenhouse gas) than nuclear fusion which basically a clean energy. The thing about terraforming is that it takes times, and resources, I believe that before thinking about terraforming it's more important to start by getting there and turning Mars into a permanent habitat for a group of humans to justify going trough a terra forming process

Btw in around a billion years the sun will be hot enough that Mars will be hot as hot if not hotter than our current earth, and with the CO2 and water melting in caps it might terraform the planet (atmospheric pressure will increase dramatical and liquid water will form) and and life on earth will not be possible anymore, heck even Jupiter moons will start to become more earth like (the ones with atmosphere like titan).