Yes and no. Cape Town does have a high violent crime rate compared to most Western cities, but the key thing to understand is that like the rest of SA the vast majority of that crime is concentrated in specific areas like the townships (slums) on the outskirts of the city. For the parts of the city and areas outside the city that a foreigner would likely stay Cape Town is quite safe and definitely safer than Johannesburg.
Opportunistic crime does exist, but provided you use the same kind of street smarts you would use in most big cities and don't do things like walk alone in unfamiliar areas after dark, don't make yourself an easy target and don't be oblivious to your surroundings, you should be able to avoid being the victim of opportunistic criminals.
As with any large city there are nicer parts and not-so nice parts in Cape Town, too. But it is a beautiful city, and I've felt safe the couple of times I've been there.
Opportunistic crime does exist, but provided you use the same kind of street smarts you would use in most big cities and don't do things like walk alone in unfamiliar areas after dark, don't make yourself an easy target and don't be oblivious to your surroundings, you should be able to avoid being the victim of opportunistic criminals.