It was hilarious. I agree with you, I don't usually like characters embarrassing themselves in public either, but once I got over my initial shock, I realized how sweet it actually was for Big Mac to go to such lengths for his little sister. The ending of the episode was perfection. Of course, many other things in the episode were great too - Applejack being referred to as "the hero of the Apple family" and "the greatest Apple of all time", everypony seeing through Big Mac's disguise except for that one poor old stallion, Rainbow Dash and Scootaloo competing together and Big Mac's amazing vocabulary. He really is much smarter than many people give him credit for. I got tired of the running gag of him not being allowed to speak a long time ago, so that's another reason why this episode pleased me. It was a LOT better than Hearts and Hooves Day, that's for sure.
I just can't help wondering where did Big Mac get the outfit and the make-up from. I seriously doubt he got them from anyone in his family, so I suppose he must have bought them in Ponyville. Looks like Carousel Boutique isn't the only dress shop in town, after all. That's good to know, I guess.
So, the episode was great fun, it had character development and Big Mac was the star for the first time. This is what Make New Friends But Keep Discord should have been like. My final score is 9.5/10.