Good question, my son.

Some of it appears to be left over from how mutate worked in Vision Design. Per Mark Rosewater:

Rather than call it out by name like champion, we made the restriction that it had to match at least one of your creature types. For flavor, we had assigned a main creature type per color. Then, when we started making multicolored cards, they got to be a combination of all creature types of its colors. White is Cat, black is Nightmare, and green is Beast, so a white-black-green creature would be a Cat Nightmare Beast.

So originally a creature being a Dinosaur Cat actually mattered for gameplay purposes; you couldn’t mutate your Cat onto a Dinosaur unless it was also a Cat.

This ended up changing in Set Design:

In Vision Design’s version, you had to match up a creature type or a keyword. The Set Design team decided that we didn’t need that restriction and changed it so that any creature with mutate could mutate any other creature.

But it looks like the once-functional creature typing stuck around and Creative must have leaned into it to give us the weirder combinations that don’t include the five main types (like Whale Wolf, as you mentioned). So while some of it is likely to blame on Creative, the germ of the idea started with Design.

That being said, Wizards does try to swing the pendulum on tone so that the overall game can appeal to more audiences, and if they aimed for the middle instead, you and I would probably lose some of the things we like, too. I’m okay with getting unusual combinations of types to enhance the silly tone of one set if they’re willing to lean into darker mechanics/creative, when appropriate (see Eldritch Moon, which I loved but some people thought was too disturbing).

I’ll ignore the Cat Apes for now and take it on faith that we get the pitch black tone I want when Magic returns to the New Phyrexia storyline.

Edit: Also: more creature types is usually better, mechanically, so at least they’re not making the cards worse with this decision.