The reason people make fun of him is because he either doesn't put research into his videos, or just ignores anything that might contradict his theories.
And he gave Undertale to the pope.

Matt was fine when he did stuff like "how much would Minecraft diamond armor cost in real life?" You still have to fudge numbers with something like that, but it's grounded and rational. Compare to "Rosalina is Peach's mom?!!" where he basically bullshits for twenty minutes. It stopped being about theories and more about throwing shit at the wall to see what sticks and brings in those sweet sweet clicks for ad money.