Honestly, they just need to up the requirements for BEING a mentor in the first place. Like a start would be so that you can only become a mentor if it would allow you access to the mentor roulette - As it is right now, you can become a mentor and not have access to the mentor roulette due to not having completed some dutys.

I do think the crown shouldn't be the mentor icon, but i do think it should still be used. Perhaps after having 5000 in-game hours you can apply the crown and you are able to put up the label "Veteran" Or something? So not really a mentor, but still an indicator that they atleast have the experience, if not the general knowledge to answer a sprouts question. I dunno, just came up with that on the spot.

The mentor icon...I dont think glasses would work really, that seems....Weird, to be honest. They're not librarians.
Watering cans could work, as has been suggested for years.
an Owl could work, usually depicted as wise or knowledgeable
Dove...
Star....

Lots of things they could choose, but glasses is just... a bit weird.