To accomplish this, simply wrap the class in a conditional expression so that the CSS doesn't show up if the config tiddler isn't set appropriately:

<% if [[$:/config/ShowFunnyText]text[yes]] %>
.my-funny-class2 {
  background-color: blue;
  color: yellow;
  font-size: 150%;
  border: 3px dotted orange;
  text-align: right;
}
<% endif %>

Note that, as discussed above, the content type must not be text/css here, or conditional expressions won't be evaluated.

The filter can be written in several ways; for instance, you could equivalently use:

[{$:/config/ShowFunnyText}match[yes]]