Yes, Rio is not the ideal location for hosting the Olympics because it has a high crime rate and does not offer the attractions and accommodations that host city should provide. Rio has not adequately prepared for an influx of visitors and without hotel or housing accommodations, people are less motivated to purchase a ticket to the Olympics.
I wouldn't go so far as to call Rio the worst host city just because they had a large string of bad luck. First of all they had the Zika outbreak, something you can't really control. Second of all, their economy went bad, something else you can't really control and corrupt officials who won't clean up the country. Rio just has bad luck, it doesn't necessarily make them the worst.
Rio de Janeiro is one of the most beautiful and nice cities, the only reason there are so many unsold tickets is because of the fear of the zika virus. This has nothing to do with the quality of the host city. It could be compared to when the swine flu was in Mexico. A lot of people started avoiding the country.
No, the fact there are unsold tickets doesn't indicate that Rio is the worst host city in history. There are many other factors that would contribute to people be unwilling to travel to Brazil, especially with the current political climates around the world and the economic struggles many people face. It is unfair to say that unsold tickets reflect only on the city itself.