Is it true for those who prefer wearing their Tzizis tassles on the inside of their beged it’s assur because the tassels touch the naked flesh of their skin?
There is no prohibition for the strings to touch the flesh. It is the common custom among Sephardic communities to wear tzitzis “in” (tuck the stings in, rather than leaving them hanging out), and no authorities have raised the issue of the strings touching the flesh.
However, one should be wary that the tzitzis should not become soiled and sweaty, for this would not be respectful for the mitzvah.
Sources: See She’eris Yosef vol. 1, p. 153.