When frothing cow's milk, if you exceed 150 degrees, you can scald the milk which makes it taste like eggs.
Is there a similar upper temperature for hemp milk?
I would follow the same guidelines as cow milk when using any alternative milk. For one you really just can't taste a liquid that is scalding hot, so you might as well not go so toasty. Other than that - If you think it tastes great, then keep doing that. If it smells less sweet, it probably is less sweet - you just gotta find that perfect sensory sweet spot. I generally aim for about 135F if it's a drink being consumed on site - otherwise about 145 togo.
Hopefully this helps!