Would construct 3 be fine for mobile games (in terms of performance) as still I have not heard of any successful mobile game made with construct 3.
Performance is good, don´t worry too much about it.
I read many articles written by Ashley regarding some updates which will make construct 3 better than 2 in terms of performance. But he also said that it will take some time. Is the task completed? Will it be beneficial also for mobile platform?
Yes and yes. Just make sure you use the construct 3 runtime. It should be default with a new project.
My only-earning will be thorough admob. Will it be fine?
Fine in terms of what? Don´t expect any substantial returns unless you can get a lot of users.
It is said that construct 3 can be run anywhere. Can I use my ipad for that? What would happen if my net gets disconnected in between?
Yes it runs anywhere, also on your ipad. Construct generally runs offline, you have to reconnect every week once though to confirm your subscription. Also obviously, if you are offline you cannot use cloud saving.
There is online support for making android apk files for construct 3 projects. Is there such support for ios? (I may be wrong)
For ios, you need a mac with xcode to build your project. The process for ios is generally more complicated compared to android.