Top off is an example of a verb phrase (dictionary.com). This verb phrase is usually followed by an object that, in this example, will be "topped off".
According to dictionary.com "top off" as a verb phrase means: to climax or complete, especially in an exceptional manner; finish:
You may have heard this verb phrase in a variety of contexts, for example:
I topped off my tank at the gas (petrol) station.
I need a refill on my drink, can you top off my glass?
In the example, "to top matters off", the object, "matters" splits this particular verb phrase. The "matter" in this case refers to an issue, affair, or business (dictionary.com). In this example in in most example of this phrase there is an issue that is being climaxed, completed, increased, or made worse.