Hi
@NoLifer of course you can create a wordpress blog / website using your mobile device. However operating it from a small device like smartphone is very tedious. It will give too much stress on your eyes and it is not that comfortable to use too. Instead, consider operating it from a desktop or a laptop.
You asked about health and fitness niche. It is a good niche but one of the saturated niches on the internet. It is very tough to rank your keywords and pages for topics in this niche as there is already a very tough competition.
Instead, you should consider any micro-niche to start a blog.