During my iddah after talaq, must I mourn and avoid adornment like a widow?

No; the duty of mourning (ihdad) is for the iddah after a husband's death, not the iddah after talaq. In the iddah of a revocable (raj'i) talaq, you are still a wife in many respects, and looking neat at home is not wrong. For the iddah of an irrevocable (ba'in) talaq, some Syafi'i scholars encourage a woman to be moderate in adornment, but it is not obligatory. What matters is guarding your aurat and the limits of mixing outside the home as usual. Also avoid displaying your private life on social media during this period, because it is easily misread and can complicate discussions about rujuk or claims. If in doubt, ask a religious officer in your district. Refer to the iddah and rujuk page.