"Jotun" consists and contains a wide variety of supernatural beings from mythology. You can read that on Wikipedia or elsewhere. :)

Some examples of jotun:

Ymir
Thor's mother
The Norns
Loki (half-jotun)
Surtr
Jörmungandr
Hel
Trolls

AFAIK Yagluth would almost certainly fit as jotun (literally ripped apart by Odin and still not truly vanquished), but his nature as fuling sorcerer-king should be given precedence, it's far more accurate definition and more practical in Valheim's context.

In short, you could very well claim as the Biomass altar's skeleton either as jotun remains, there'd be little way of contesting your stance on that.

We could also take a look at what creatures of Valheim are likely not jotun/jötnar:

Eikthyr (https://en.wikipedia.org/wiki/Eik%C3%BEyrnir)
Elder (shoot of Yggdrasil)
Biomass (undead abomination)

And finally, since even the World Serpent Jormungandr can considered to be jotun due to family relation (offspring of Loki and jotun), we can't really be sure about Moder and the Queen:

Moder (frost wyrms are "most ancient kins sprung from Ymir's body", implying close relation, which is important when considering nature of jotun)
The Queen – too little information available, could be nothing else than a mere "beast" similar to Eikthyr