Yes, you're right!
There's serious voltage drooping when the oscillator board is connected to the AD9957 evaluation board. The oscillator(16.368MHz) I use is from RAKON. The attachment is its datasheet. The following snapshot is from the oscillator datasheet.
And another snapshot about the REF_CLK is from the AD9957 datasheet.
Maybe these two snapshots could privide some help. I don't know what the load resistance mean.
Best Regards!