"they just evaluate their level of trust with the source" — No. They evaluate it but how well it fits with their preconception and other ideas they already hold EVEN if it comes from a completely unknown source. Memes are usually part of a "memeplex" and reinforce one another. And the most powerful memes also undermine competing memes at the same time. (for example, the "don't trust the MSM" meme works well with "the election was rigged" meme.