Is it Ok If I MIx Articles That is Not Relevant to My Blog Niche ?

No one is going to tell you that you can't because of course it is possible. However, if you are serious about ranking well, increasing traffic and earning from your site, I wouldn't recommend it. Sure, you can increase backlinks and possibly your search engine ranking, but you also risk driving away your faithful readers. You know, the ones that visit every day to see what new content you've posted. If you have a fashion blog, likely your visitors are coming because they are interested in fashion. If they want to learn about money making opportunities they would just visit a blog centered around that topic. Unless of course, you can find a way to integrated the two topics into one post and make the money making topic relevant to the overall niche of the blog.

You can mix it my friend as far as I know because the niche is really there and whatever you post. Those searchers will still type what they need to find on the net that is the used of the niched or keywords... Did you configure your meta tag, my friend?

As soon as I read the title of your question, the first word that came to my mind was a big big "NO". It will do no good to your blog whatever efforts you take. There is a niche as the word you used. So, stick to it. Don't stuff your blog with those things that are not under that topic. What interest would it generate for visitors if they get to see sports news when your blog is dedicated to making money? Think from the perspective of a visitor. Would you like visiting such a blog yourself? Instead of having all types of articles in your blog, spend your time and energy on researching the topic of your blog if you so think. That would show your passion and bring in more visitors instead.