To say that Yo La Tengo can't sing is merely a sign of narrow-mindedness; it's like saying that Pollock couldn't paint. It’s not about whether you appreciate Yo La Tengo or not—they can be as awful as any other artist—it’s about understanding LANGUAGES. In the genre that Yo La Tengo creates, vocals don’t play a dominant role as they might in easy listening or Sanremo music, and even in mainstream rock, Yo La Tengo embodies a culture that sees the voice as one element among others, not the defining element of the proposal. Ever heard of shoegaze-pop/dream pop? There, understated vocals and dreamy harmonies are part of the genre, so it makes little sense to compare the singing style of a Bonovox with, I don’t know… the Pains Of Being Pure At Heart (who, by the way, I don’t even like); they are completely different styles, and the characterization of the style reaches into the role that the voice plays.
Now, Yo La Tengo can’t be regarded as a shoegaze band per se, but their singing style is a blend of shoegazing-dream-pop and the VU. This should be understood; then you can appreciate it or not. But saying "I hate cubist style" is one thing; saying "cubists can't draw" is another. One could argue that Michael Stipe's voice pales in comparison to Pavarotti's, and it’s undeniable that Pavarotti sings much better than Stipe, but perhaps that comparison itself is foolish, as it only reflects a narrow viewpoint. You can't judge the skill of a pop-rock singer with the same criteria you use for an opera singer; it’s a different thing, a different world, another language, another culture. Similarly, it makes little sense to criticize the vocal prowess of Yo La Tengo, where the very concept of vocal prowess almost clashes with their expressive style.
Besides, it seems to me that bassist James McNew has a nice little falsetto with a soul inflection—not bad at all. Kaplan has a "friendly" voice, nothing special, a pleasant timbre, he is in tune, but often he doesn’t even sing; he adopts a sung/spoken style reminiscent of the Velvet Underground. Georgia Hubley presents herself with a minimal, hushed style, but her timbre is intriguing, I would even say quite sensual. I can also add that sometimes I would prefer to hear her voice recorded a bit louder, because that would just be a simple adjustment, and I think some songs would benefit, even if they might lose something on the hypnotic tone, but… they like it that way, and that's that. When they sing together, I find it very pleasant, in my opinion, but tastes vary. Ultimately, it’s fine to despise, but first you should understand WHAT you’re listening to.