Your mother is so fat they assigned her...

Your mother is so fat they assigned her a new top-level domain.

This joke is both a your mother joke and a programming joke. To understand it you have to understand what a top level domain is.

Top level domains are the part of a website url that comes last, for example "com", or "org". They are usually assigned only to countries or to large groups of related sites. There are a few single companies that own top level domains, but there are not that many, and the ones that do are very large companies.

For a person to be defined a top level domain, they are being compared to massive companies or countries in terms of size.

Basically, they are saying that your mother is very fat.