Assuming inter-universal travel is possible, can a perfectly
mathematically describable universe exist inside a multiverse?
I would actually say no. Assuming inter-universal travel is possible, there may be sources of randomness coming in from other universes. Namely, the multiverse could influence the universe, making it unpredictable.
For example, you're observing one universe and have built a satisfactory model to predict its behavior. Then some aliens unexpectedly visit that universe from a different one that you weren't observing. Bam, your model is broken.
If it could, would that mean that the multiverse is mathematically
describable as well?
Okay, this is where things start to get a little wonky. Per my response to your first question, I don't think you can create a "perfect" mathematical model of anything within an unpredictable system. There's always the chance that the system will do something you don't expect to the thing you're observing.
However, if you have a perfect mathematical model of the system, you intrinsically have a perfect model of anything and everything within the system. Putting this another way, you can have a perfect model of a universe if and only if you have a perfect model of the multiverse it resides in.