BASIC has its roots as a teaching language; its design favored a shallow learning curve over the powerful expressivity and terse syntax that are typical for languages aimed at "professional grade" programming.
Visual Basic, then, was initially based on the idea that anyone should be able to program if only the language were easy enough. Microsoft promoted this idea, and many businesses picked up on it - after all, the idea of getting rid of those pesky programmer types and just doing the same 'easy' work yourself meant you could save truckloads of money and know exactly how the software worked.
We all know how this ended, and as a result, Visual Basic carries kind of a stigma, being the language that incorporates this horrible failure. That doesn't mean it's an intrinsically bad language: modern VB.NET has much of the same semantics as C#, and it can (or at least could) do a few things C# can't.
The language itself is not what people are making fun of anyway: it's the culture that surrounds it, or rather what that culture is believed to be like. Whether the mocking is justified is a different story.