What logical fallacy is this?

[Tyshalle]

What's it called when somebody tries to shrug off a point one of your sources made by saying "nobody takes him seriously," or something to that effect?

[Oskuro]

Probably Ad-Hominem, attacking the integrity of the speaker rather than the arguments, although it could have a bit of appeal to popularity/authority thrown in.

[Darth Wong]

What's it called when somebody tries to shrug off a point one of your sources made by saying "nobody takes him seriously," or something to that effect?

That depends on whether it's a "point" or a factual claim. If it's a factual claim and the source is not considered authoritative, then it is actually reasonable to dismiss it by saying "no one takes him seriously", assuming that's actually true. If it's a logical point, then you should learn it well enough to express it in your own words, and then it doesn't matter if no one takes the original source seriously.

[Tyshalle]

It's in reference to a movie review. I posted a link to a video movie review saying that I largely agree with the points he makes against the movie, and somebody replies, rather than by arguing any of those points, instead by saying that it got a 95% rotten tomatoes score for a reason and that nobody takes the video reviewer seriously anyway.

[Thanas]

That may be an appeal to popularity, but without the exact wording of your exchange it is a bit hard to tell.