Are converts expected to be more observant than the community they are joining? For example, if most married women in a community don't don a wig or head scarf, would this be acceptable behaviour for the convert as well? If a convert was super-observant during the conversion process and then after their conversion gradually became as relaxed as the other members of the community in terms of observance, would their conversion be invalidated? I guess the gist of my question is, is a convert expect to assimilate to the community they are joining, or are they expected to be super super observant, even if this means exceeding the norms of that community?
It's not surprising that if someone's lifestyle is being evaluated, that a higher standard is expected. This has been frustrating for many a convert (as well as born Jews who've affiliated with a more-observant lifestyle).
In theory, conversion is instant and irreversible. One second before converting, the would-be convert could eat pork all s/he likes. One second afterwards (if we magically knew the convert was totally sincere at the time of commitment), there could be all the regret in the world, still, no pork allowed. A rabbi could meet conversion-candidate "John", who is 100% absolutely sincere about committing to keeping kosher. But given how unstable John's lifestyle is (e.g. he lives on a college campus with no kosher meal plan and zero other Jews in a hundred-mile radius), John is likely to stop keeping kosher soon. If so, the rabbi may not want to convert John as he'll just be causing him to sin.
Rabbi Moshe Feinstein ruled that conversion requires "sincere intent to observe all the commandments", with the caveat "as best the candidate understands it at the moment." Someone asked him -- "when I converted, somewhere in the back of my mind I knew that if I was threatened with being fired rather than work on Jewish holidays, I probably would have gone into work that day" -- well, people make serious commitments all the time that are tested -- Rabbi Feinstein ruled that conversion was still valid.
Similarly, regarding an individual who recently converted in a Sabbath-observant community where married women don't usually cover their hair (outside of synagogue) and many lay people eat fish-baked-in-foil at non-kosher restaurants (I do not condone this practice), Rabbi Aaron Rakeffet-Rothkoff commented that her conversion was valid even if she did likewise, as she accepted Judaism to the best of her understanding -- and her understanding was based on what she saw her Jewish colleagues doing.
Lastly, the Talmud says that converts can make things difficult on born-Jews, with one explanation being that the sincerity of their practice puts "the mainstream" to shame!
As a convert myself, I can tell you that the newby convert not only seeks acceptance, but worries about rejection; some more than others. As a result, it is a natural tendency to err to the strict side. I would add that converts of color and others whose physical characteristics makes it impossible to "pass," tend to be even more strict in their observance and dress. This tendency long precedes my experience and is recognized in the Talmud. There is a Gemara, Pesachim 91b, which banned a group consisting only of converts from bringing their own Passover offering. The idea there was that a group of converts only would be extra-strict and might mistakenly find a defect in a perfectly good offering. (But see Tosafos at Pesachim 91b Who points to other views that converts are less stringent.) There are numerous comments, as well, on Rabbi Chelbo's comment that converts are like a "scab," some interpreting this to mean that converts can be so strict that it can be embarrassing for Jews from birth. See Tosafos to Kiddushin 70b who quotes R' Avraham the Ger who said that because converts are more meticulous than born Jews, they underscore the shortcomings of born Jews.
In a review class about conversion, Rabbi Daniel Stein said that a conversion is invalid if the convert did not intend to accept all the commandments. Figuring out from later behavior what constitutes a failure of intent to accept, as opposed to later sinning is difficult. He suggests there that activities within 30 days may be grounds for invalidation of a conversion. He also says that there is no requirement that they think they will never sin. So if a convert speaks some lashon hara (presumably even within 30 days) that does not invalidate the conversion.
Applying this to the case discussed by the questioner: If a convert says, "I have no intent of covering my hair when married." that would be an invalid conversion. If on the other hand after a significant period if she stops covering her hair due to social pressure, or even if she never covers her hair, assuming she doesn't marry until some time after conversion, she might be a sinner, but she is still Jewish.