I assume that you are referring to informal use of English? So, 'looking good' actually refers to complimenting somebody about their looks. In certain cases, the things that the individuals are doing, can be, in present tense, complimented as 'looking good' too. Although, that would be less heard of. Whereas, 'looks good' can be used on just about anything, from humans down to inanimate (non-living) objects. Both have almost the same meaning, and in most cases, not even be brought up for a correction.

Thank you so much for such detailed explaination. I know it better now. (^_^)Y

Hi, looking good can be used for people, such as the comment you saw, you can say it to someone in person as kind of a statement, definitely informal a little playful, for example if a friend got a new haircut, you normally nod your head up and down when saying this phrase either to a person or object. Lets say your neighbor is painting their truck, you could walk up look at the truck nod head up and down toward truck while you say... Looking good. When looking good is used at an object it is not always informal, you could look at a financial report and say, looking good. Looks good is normally used for an object, for example, that hamburger looks good! Or if looking at cars, that car looks good. You would more commonly say to someone " you look good" rather than tell someone "she looks good" (referring to another person). An example of using the phrase looks good for person would be pointing at a resume of a candidate for a job and telling someone "he looks good." Referring to his qualifications of course, not his looks because resumes usually do not have photographs attached.