Yes. It is totally safe for womens. Thousands of female travel Rishikesh every year, both from ( India & Out side India). You need to take care of few things - Be sensitive near holy places, Never Carry alcohol or Non Veg food. Although Alcohol is banned in Rishikesh, In case you are manage to arrange something. Never Ever try to drink Near Ganga. People are generous and helping in nature. Stay in Camps or cottage that have some name. ( Although all the camps are safe) Never ever argue with staff on hotel, Camp or cottage. If you have some complain ask the manager or owner or the agent you have booked through.
These are safe places to travel solo. but you need to be good on time management as the tungnath trek can last for few hours, depends on how much time do you take to complete the trek and how soon do you come back
Now most of the travel destinations are safe for women as millions of women are travelling solo but again in my opinion we should always be careful with the surrounded people as social elements are everywhere in every corner