Shall I tell my college professor that the reason why I am asking for a letter is I have to have a letter from college? I have graduated for almost two years so I have many other stronger referees; my college performance was not stellar. The only reason why I am requesting a letter from my undergrad profs is that the program I am applying to require one letter from college.  Do I suppose to tell the Prof this reason?
It isn't essential to give this information. Hopefully one of your profs will be happy to write you a letter. If you ask more than a couple of people and they seem reluctant, then you could mention it, but it probably won't result in the best letter possible. 

But first choose someone with whom you did well and who might have a good memory of you. Ask as usual, and thank them as usual.