I've started to notice that when I deliver a cake and there are other people who gather around to look at my cakes, there is always someone who makes a comment such as: "oh, I work in a bakery or grocery store". " I could have made a cake like that" and ....... "I don't like Fondant, I like only buttercream".....AND.......then they start to really really "critique" my cake or cakes in front of everyone else or directly in front of the client.
I get so mad when this happens, that I want to say to them, "if your going to critique my cakes, at least wait until I leave the room"..... 
Then what happens next is the client who ordered the cake(s) will get into a huge debate with the person or persons "picking" about my cake(s) and the client will tell "that" person, "well, that's why I don't get cakes from where you work".....
I also had someone who told me, "well, at least you've leveled your cakes nice", and "clean flower's"....or want to know what type or how did I make my cakes and or icing etc....
I don't give them that info, because I figured, that if they work in a bakery or grocery store, they are only picking my brain to either change or improve their recipes.
Then I had someone on my delivery cake to the client, "this" person immediately tried to grab the cake box and then proceed to tell me, "Oh, I know how to carry cakes, I've done it before". And when I tell them thanks but no thanks in a more polite way, they get angry at me as if I'm acting like a snob. Well, call me dumb,, but I don't want my cakes ending up on the floor.....
But what makes me really laugh is when after I go through the picking on the cake field,,, and them telling me, they only eat certain types of cake etc.. and the client will call me afterwards to tell me my cakes were great and there was nothing left, that everyone had 2 slices,,, well, that just makes the bubble look better !! 
I have people ask me how I do things,but have never had someone critique or say, "I could have done that". I can't believe how blatantly rude people can be sometimes. Ignore it as it's the customer who matters after all.
Amy
Before I really started decorating, I made DD a castle cake for her birthday, pretty simple and not great, but I was proud of it. One of the moms who was a GUEST in my home at the party, went on and on about the castle cake she had made the year before that was so much better than mine.
People can act so ugly! 
I agree....don't let it bother you. In over 29 years of doing cakes....you hear allot.....and I just set my cakes up, take the pics and then head out. One time I did get a little bit tired of this one gal going on and on about how she could have done the same cake.....and I finally just said...."Doesn't look like your cakes are here, are they?" And the lady who had ordered them....started laughing. So don't let it bother you...there are always "wanna be" cake critiques out there.
That happened to me yesterday, I delivered a cake to my BF in another town, and her sister started in about how I should get 'a thing to smooth the sides of the cake better'....it was cream cheese icing and I smoothed it almost perfectly....she is an artist too and she kept telling me about all these things she designed for HER cakes....
I just wanted her to but I wasn't gonna say anything in front of my friend....I figured if her cakes were so much better than why was my friend paying me to drive 45 miles to give her a cake??!?! Right???
All I have to say is that people can be so rude! They either think they know better and correct you all of the time or talk about you all of the time (even in front of you). I guess if it makes them feel better....who cares how the other person feels!
I would just say "Yes many people can make beautiful cakes. I'm just so thankful that people are willing to pay me to make them one."
Sort of a dig that YOU are the one who got the job and not them. Like if they did such good cakes then why were they not the one got paid.
I had a woman do that to me once and I very sweetly smiled at her and said "well you're always welcome to do better." Smile smile, bats eyes coyly... 
You could always say something like, "oh, apparently since you are so much more skilled than myself, can I see your portfolio?" They would get your nice sarcism.
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