Do Corner Bass Traps have to be Triangles?

I'm confused by your question, but basically no the shape doesn't matter at all. Just the amount of absorbing material and its depth, and where it's placed.

I don't really understand how it's shaped as a triangle though...?

Many of the buy it off the shelf bass traps sold are triangle shaped and they tell you to put them in the corners, so I can see where the OP got the idea from.

Room modes (standing waves) can occur between parallel walls, between opposite wall corners of the room (e.g. front left and back right), and between opposite wall-ceiling-floor tri-corners (e.g. front left ceiling and back right floor). Or combinations of the above.

What all of these have in common is that they are all possibly present in a room tri-corners. (e.g. the front-left-ceiling corner).

Because of this, corners (three surface corners or two surface corners) are favored spots for bass traps. You can tame the most modes there.

Also, cheap fiberglass bass traps require a lot of depth to be effective. A 12" deep bass trap on a wall is hard to hide. But if you pack fiberglass into a corner, the deepest part can be 12" deep and the whole thing doesn't stick out into the room much. Another reason corner spanning traps are favored.

I suspect (without testing data!) that being close to the corner and absorptive of a broad range of deep frequencies is more important than the specific shape. If you can get 12" deep material flush up against the corner, standing waves are not going to be able to use that corner for reflection (one of the walls will be damped).

Go for it!