I am accepting @hazoriz's fundamental explanation that the word מקבל means "accept", as a foundation for my expanding on this. The next part of the phrase says את כל האדם which means "all of the person".
I heard this great explanation from my Rav when he was teaching his summer Shabbat Pirkei Avot lesson. It says The person, and not just the word אדם, meaning "person" - that's the word we would expect to see here. By saying the person, it means that that one should view all of each person's character as positive, even if he has flaws, which may be starkly noticeable. Even someone's flaws are to be accepted cheerfully. (Related to this, BTW, is the phrasing of the bracha "Borei Nefashot" where you praise G-d for creating flaws.)
In order to accept all of the person, you have to, of course, greet and meet each individual. As stated, in the above comments, perhaps, in the general context of a large crowd, Gedolim don't appear "cheerful" when addressing the crowd.
However, from personal experience, I met the Lubavicher Rebbe a few times. Even though there was a line of people, I know that he smiled and was cheerful to every person he met. And, BTW, you can see him on several youtube videos, where you can see him greeting everyone cheerfully - he doesn't necessarily need to smile with his teeth showing to make you realize that he is "cheerful". And, even in cases where people have told the Rebbe about "flaws" - they sometimes tell him about things they feel they did wrong, I have seen him (at least on several of the videos) still accept the whole person positively, in support of how I have explained the Mishnah.
Rav Binyamin Kaminetzky, whom I used to see quite often, not only always had a great smile, but had no problem accepting some of my "unusual" religious practices and choices, perhaps what he in his own practice would consider a "flaw". It was never an issue to him.
Conclusion - based on my explanation of the Mishnah, I can't find any justification for anyone - not just Gedolim, to not treat all of the person cheerfully.