Myself, I have a lot of SJ / SP types in my life, so I feel a need to frequently stick up for them here on the boards. When people post stuff like "I could never date an SJ" or "they're not N so they're not deep enough for me" it drives me crazy! Or that F's don't think, and T's don't feel - what nonsense!
When posts and comments fall into cookie-cutter stereotypes, it benefits no one. People are so much more than a composite of 16 "types". MBTI has uses, and I find it very clarifying and validating even, but making judgements about others based solely on it is very dangerous.
That being said, I do attribute some of what I see here from time to time on life stages.
How ironic is it that there are at least some NT's judging these SJ's as a group using feeling as a judging function and neglecting the thinking judgements that they're usually so adept at.
There are definitely a lot more N's here on the forums than I know in real life. Also, the INTP population on the internet is staggeringly high. I know very few INTP's in real life.
Anyway to answer the original question, no, underrepresentation leads to a negative perception. No one really has a negative perception of ESFJ's or ESTP's. I think we all understand that each type has its positives and negatives.
I do wish there were more of the underrepresented types around here (EJs in particular). But if people want to make fools of themselves my spewing hate towards whole swaths of people they don't know, it doesn't really reflect upon the faceless hordes.
More balanced numbers would make conversations more knowledgeable and productive though.
I'm tempted to say everyone on this forum - even the most commonly represented - aren't great representatives of their types as a whole.
We're like a sub grouping of our respective mbti types who are super into psychology and intra- and inter-personal analysis. There are a helluva lot of people out there who aren't into internet/forum stuff period, and aren't preoccupied with psychology, and are busy with their lives and hobbies.
"...On and on and on and on he strode, far out over the sands, singing wildly to the sea, crying to greet the advent of the life that had cried to him." - James Joyce
Mmm, I think most people are interested in psychology and intra/inter-personal analysis, just a lot of people are more pedal-to-the-metal, actually just doing it rather than all this meta-analysis with all the words and jargon. Which is fine.
I think it would be really draining and unreasonable to expect anyone to represent a huge entire group of people. (Let alone try and defend and "speak for" a whole group... which is what I sorta understood as to why Sarah left. )