Is it a good idea to cap fiberglass batts with blown cellulose?
It's the opposite, in fact: in terms of thermal performance, cellulose is better on the top layer. The reason for this is that compared to fiberglass, cellulose is more dense and more opaque to infrared radiation than fiberglass is. Most attic heat gain is caused by infrared radiation, and convection through the insulation plays a strong role in heat loss. Having a product on the top layer that's denser and more IR-opaque helps to prevent convection within the insulation and slows infrared radiation more.

Cellulose is fire-resistant because of the borates, but by no means fireproof. It is still paper. I've heard of attic fires caused by smoldering electrical work done wrong but buried in insulation. Fiberglass of course is basically fireproof until the fire is so hot that it causes the fiberglass to melt, which is somewhere around 1,100 degrees Fahrenheit. Much less risk.

A superior product compared to both is mineral wool. It's got all the advantages cellulose has over fiberglass (in fact, it's even better), but it's inorganic and thus fireproof. You'd need to install this product in batt form, since it doesn't generally come loose like cellulose and fiberglass. And it is much more expensive than those. But if you want optimal thermal performance as well as fireproofness, it's hard to beat.