Going around in his actual form would definitely draw some eyes and perhaps some horror for those not more curious than reactive. And that could be more of a struggle than what it is currently. Istredd would likely take a human facade if he was something inhuman, just to avoid the drama. But his concern is for Lucifer's sanity.

"You could always see it as a for now situation. You're ancient, you have time to figure your thoughts and feelings out." Although Istredd doesn't know if him being locked into a human form means the body can die from age. He's assuming not, otherwise that might be something more to angst about. It may be preferable to get all the answers right away, but not logical. It is his fault bringing it up though that it's probably a mental block, and he's right.

"The heralds were amplifying all the worst things. I could put an illusion on your face if you want. I can make you look like anything to yourself." Istredd was taught by the greatest illusionist in the Brotherhood, his skill with it is expert, he just doesn't use it as openly. He did for Stephen, when they were doing the cards, creating an elaborate illusion in a second's notice to hide them as they worked.