What's the intent of the notice?
If it's there just to attempt to get people into a respectful state of mind and demeanour and to behave appropriately, then I think "the next exhibit features human remains" is sufficient.
But if it's intended to give an idea of the nature of what's there, let people know that the next room has certain exhibits that not every visitor may feel comfortable viewing, and give people an opportunity to make an informed decision on whether members of their party might find it distressing, then yeah, I agree that "human remains" is a bit too vague to fulfil that purpose adequately.
Not everybody can be assumed to have the existing knowledge or understanding to know precisely what to expect in that room. "Human remains" can be things like ashes in a jar, a single finger bone barely visible behind glass, an unrecognisable dried-out or preserved organ, a whole wrapped mummy complete with mask and accoutrements, ancient remnants of skeleton mixed up with grave goods, or sometimes a full desiccated corpse with face visible.
(I once came across the last of those completely unexpectedly, in a small Italian church. "Oh this looks like a nice church", "Ah isn't this bit beautiful", "It's so calm and cool walking through here", BAM surprise corpse display case. I should probably have read the town guidebook first, in retrospect.)
And many people who could cope with a murky finger bone, or a jar with a sign saying there are human parts inside, or a fully wrapped mummy, might struggle with seeing a dead human being, skin stretched taut over bones, and for a lot of reasons, may not have realised that they'd be seeing that. It could be the first dead person they've ever seen.
So really to me it depends what they were trying to do with the sign.
If it's to signal that respectful behaviour is expected in the next room, it's fine.
If it's intended to provide useful information to help people choose whether the display will be appropriate for their party, it's only useful if you have such a level of preexisting knowledge about what's there that you probably don't really need the sign.