Having sex with anything that wasn't human or monster girls and based on real-world animal counts as bestiality, broadly speaking.
Although, strictly speaking these girls are upright animal (furry scale 2) with some differences, like humanlike tits and vagina hence bestiality tag is still correct.
If it involves a satient being, it's not Bestiality.
That's why it's called "Bestiality" from the word "Beast", non-satient being; an savage animal.
With that logic, you can say that having sex with a Pokemon is considered Bestiality (although a lot of people disagree with this statement because they are, well, as they really want to impose, "satient beings").
Short answer: No, it's not Bestiality.