Was there much point to showing her old and wrinkly again?
She took off the necklace to reveal her "true self" during a serious crisis of faith. She'd invested all of her belief into Stannis' cause, only for that cause to fail pretty spectacularly after she burnt a child alive. She'd identified Jon as an alternative during s5 because she detected something special within him, but upon her return to Winterfell she found him dead on a table. She even says herself to Davos, "The victory I saw in the flames, all of it was a lie". At the beginning of s6 she's doubting not only her powers but whether the Red God exists at all, and in the end it's Davos who has to talk her round.
Remember that scene from s5, where Jon asks Mel whether she's cold considering she's only wearing her dress at Castle Black, and her response is to say the Lord's fire keeps her warm and to press his hand to her cheek? She spends all of s6e1 in huge scarves and winter clothing because she's hit rock bottom so hard that she no longer feels her god living inside her. Mel's old and wrinkly form represents her giving in to her very lowest point. Her faith is somewhat restored after Jon wakes up, and once the Stark banners unfurl at Winterfell they cut right to her because it's her vision that's come true.
There's even the scene before the battle for Winterfell with Mel and Jon in the tent discussing her faith and his resurrection, and in that scene she admits she has no idea what the Red God wants from her, that she's a conduit and nothing more. Once the battle is over, Davos demands she be executed for burning Shireen and Jon gives her a chance to defend herself ("Do you have anything to say for yourself?"). Instead of defending herself she just admits she's been ready to die for many years, which is a pretty concrete reference to the fact that she's old and wrinkly underneath.
There's no need to bring it up again because her crisis of faith has been dealt with, and now she's weaving the Lord's web in Daenerys' direction. Maybe if she dies at some point she'll turn into her old and wrinkly form before she gives out her last breath, but until that moment it doesn't really make sense to do it.