The basis for celebrating a wedding has, in my understanding, little to do with the mitzva of tznius. The same is not true regarding the celebration of a bas mitzva.
“Actually, the mitzva that the gemorah says requires Tznius is a wedding (once again, Succah 49b).”
This Gemorah seems to confirm that tznius is not inherent in the mitzva of marrying, only that getting married must be done with tznius.
The focus of a chasunah is the man’s obligation to marry; the focus of a bas mitzva is the girl’s obligation to perform her mitzvos, of which tznius is the core.