Short answer for most visitors: yes, with normal big-city common sense. Here is the nuanced version.

Violent crime against tourists is relatively uncommon in major destinations. Petty theft can happen in crowded tourist zones and transport hubs — watch bags and phones.
Be cautious with unsolicited approaches around major attractions pushing tea ceremonies, art shops, or 'student' stories. Smile, decline, and leave. Use official tickets and well-reviewed services.
Drink bottled or boiled water. Air quality varies by city and season — sensitive travelers may bring masks. Pace yourself in summer heat and winter cold in the north.
Not because cities are dangerous — because navigation, transport choices, and situational translation reduce stress. Solo travelers often value this more than couples.
Central areas of major cities are generally busy and fine; use normal precautions and official ride-hailing.
Busy restaurants with high turnover are the usual strategy. Communicate allergies carefully.