Humans from Pokémon Universe are Pokémon, but they are not biologically closest to Pokémon like the Ralts or the Machop line at all. They are the most far removed group from all the rest, and here is why...
Humans from the Pokémon Universe are necessarily Pokémon themselves. Every life form is a Pokémon pretty much, and all references to real life animals were erased with the gen 3 soft reboot.
This means humans are a kind of Pokémon and this was confirmed in gen 4.
But does this mean they are biologically closest to primatelike Pokémon, especially to humanlike Pokémon like the Ralts or the Machop line ?
No. They are not.
Humans are descendants of Mew, a Pokémon who likely evolved naturally from an abiogenesis event like real living creatures, but they are from a lineage that separated before all the others.
Why ? Because all Pokémon, excluding some major Legendaries who are the "gods" of the verse and were created by Arceus, reproduce by eggs. Some Pokémon, such as the minor Legendaries, evolved to be very long lived but lost the ability to reproduce altogether, and are now present in extremely low numbers, hence the Legendary status. But 95% of Pokémon lay eggs, and the other 5% do not reproduce, except for Mew itself.
Humans, on the other hand, give birth to living offspring, just like Mew itself.
They are the only group that never became oviparous.
Hence they need to have separated before the other direct descendant lineage of Mew and common ancestor of all other Pokémon evolved oviparous behavior.
Humans are also the only group that lost all moves, but many Pokémon have like 2 or 3 moves in the whole movepool already.
Mew is basically an embryo, a complex organism with comparatively few cells. Except it is over 1 feet tall, making its cells quite huge.
This means it stands in for creatures that emerged about 600 mya. By 520 mya, Anomalocharis and Trilobites were around, and so were the Anorith and Kabuto lines. This means humans separated from Mew between 520 and 600 mya.
Pokémon such as Primape, Slaking, Oranguru and even more Gardevoir, Gothitelle and Conkeldurr evolved CONVERGENTLY to humans, but they are not human at all.
So a lineage of Mews that never became oviparous had to evolve into a ratlike animal by about 225 - 280 mya, then by 80 mya into the first primatelike forms, into actual monkeys by 40 mya, into apes by 25 mya, into hominins by 7 mya and into mankind by 2 mya, and finally into the current iteration of humans by 300 kya. Meanwhile, they became Normal types and lost all their moves, and the Ability Syncronize was replaced by nothing, or pheraps by Adaptabilty.
I calculated that humans have 50 BST points across the board. The same as Snorunt, even though Snorunt is just a random Pokémon. As shown in a recent Pokémon movie, they can actually gain back the ability to learn moves sometimes. Normally they do not have any and our physical attacks are not moves since Pokémon moves are powered by elemental energy and we do not have that.
Most humans are level 1 to 2 or 3, with some very powerful humans such as Ash or Jessie and James being level 5, even if with some occasional PIS feats.
We can determine they are not actually stronger than that because even initial starter Pokémon are portrayed as strong enough to protect humans from threats.
It also seems humans can not train and reach level 100, except for 1 non canon, extremely ridicolous case in the Pokémon Manga, where a human was 1HKOing fully evolved Pokémon in their 40s, tanking their attacks with no significant damage, and was also fighting on par with Level 70 Regigigas and Deoxys, showing the power of a low BST Level 100. Pheraps they can go up to level 10 if they train to go past level 5.
So most humans would get stomped by Showdown Little Cup Pokémon.