the more adequate translation would be “the weirdest thing”

“Tout ce qu’il y a de plus [adjective]” is a set phrase. It is an intensifier for the adjective, meaning that the quality expressed by the adjective is valid to a full extent. It can be translated by “as [adjective] as it gets”, or as maxmax suggests “the most [adjective] thing”.

Grammatically, start from “ce qui est le plus bizarre”: “that which is the strangest”. From there, move on to “ce qu’il y a de plus bizarre”: a set phrase (where bizarre is one possible adjective among many), literally “that which there is, which is the strangest” or “that which is the strangest among what there is” or “the strangest thing among what there is”. Adding the word tout in front acts as an intensifier for the fact that the adjective applies: “that which is the strangest in all aspects among what there is”.

The construction is an absolute superlative, with the connotation that the intensity is as high as possible. As a superlative, it does not insist that the object under consideration is the most bizarre, as in more bizarre than the others. Etymologically, it expresses that it would not be possible to have something more bizarre, without precluding that other things are as bizarre. Because the expression acts as an identifier, this literal meaning can be hyperbole. In fact, the idiom means that there is no doubt that the adjective applies, that the adjective applies by anyone’s standards.