Did the Buddha teach receivers to be ungrateful?
The Buddha taught people (especially monks) to be &quot;worthy of offerings&quot; -- that's in the Saṅghānussati for example, which also says that such a recipient is &quot;worthy of respectful salutations&quot; and  &quot;a field of merit&quot;.
Apparently that (being &quot;worthy of offerings&quot;) does not imply fawning on laypeople in gratitude.
On the subject of fawns though, AN 2.31-32 does talk about gratitude and especially in relation to parents:

Now what is the level of a person of no integrity? A person of no integrity is ungrateful &amp; unthankful. This ingratitude, this lack of thankfulness, is advocated by rude people. It is entirely on the level of people of no integrity. A person of integrity is grateful &amp; thankful. This gratitude, this thankfulness, is advocated by civil people. It is entirely on the level of people of integrity.&quot;
&quot;I tell you, monks, there are two people who are not easy to repay. Which two? Your mother &amp; father. [etc.]