The beholder called them "mom", so I'm guessing it's one of those "created them from their own dreams and raised them" scenarios? Or from a friends dreams, but they 'raised' them?
Also a 'ooh totes evil creature by nature' like a beholder not being completely evil fits with OoTS style.
They're born from the dreams of other beholders though, not just any creature. And generally in a fully-grown state, as it's basically a manifestation of the dreamer's concept of a beholder (such as themselves). Looking it up though, there does appear to be a slightly more "conventional" form of asexual reproduction available to them, and seeing as the progenitor either eats or rejects all but (at most) one of the brood, that would open things up for being discovered and raised by our good friend Serini.
...however, this doesn't necessarily dodge the issue with their temperament.
It's not that beholders are "Alignment = Evil, therefore bad", it's that they are
specifically described as being singularly xenophobic, narcissistic, and paranoid. So, basically all the traits one would look for in someone you want to befriend.
Beholders are perfectly capable of being not-diabolically-evil. They'll just hole up somewhere remote and keep perpetually building and reinforcing their lair's defenses in preparation of the imminent attack from all sides that is totally going to happen, because nobody and nothing can be trusted. But cooperating with anyone that they don't view as either a close enough representation of themselves (the perfect entity) or a slave they can keep contained and controlled is... Kinda the one specific thing that they're made to not do.
And yes, there are indeed a great many varieties of beholder or beholder-kin. Some, like observers, are indeed noted as having less-antisocial personalities than true eye tyrants.
...and just about all of them are readily distinguishable from eye tyrants by their body features. Sunny isn't an observer, because they only have one central eye + mouth. They're not a spectator either (specifically mentioned as being "known to form friendships with other creatures, a trait that no other beholderkin or true beholder ever displayed"), because they have more than four eyestalks.
Curiously, though... Sunny doesn't appear to really be
any variant I'm familiar with. They possess all the characteristics of a standard eye tyrant, except that they only appear to have eight eyestalks instead of ten.
I suppose it's theoretically possible that a true eye tyrant dreamt of a "wrong" version of themselves that was possessed of disgusting and weak qualities like empathy, respect, and deference (and additionally being impure of form enough to only have eight stalks), but that raises the question as to how it wasn't immediately slaughtered by its dreamer as an abomination. I guess she just showed up at the right place at the right time, immediately understood that this new beholder was different, and managed to imprint upon it and scurry away before the original woke up or just backstabbed the hell out of mommy dreamest before making away with the kidball.
So, really, the only reasonable explanation so far is that this is indeed O.C. Donut Steele.