Can dust build-up in Time Capsule (vertical model) be a fire risk (or other risk)?
In terms of being a fire risk, no, dust building up inside a Time Capsule is not a concern.

Depending on the hardware, dust can be a concern for other reasons. In the early days of computing when floppy disks were the main storage medium, we used to have a saying that getting dust onto the disk was like running into a brick wall - not good at all.

However, I have worked on countless computers over the past three decades and they're usually full of dust inside (well, in tower models at least). None of these machines have ever caught fire. 

With regard to a Time Capsule, they do not generate a lot of heat - much less than a computer with multi-core processors doing some heavy processing tasks. So you can rest easy.