If a data DVD has a scratch but is readable without any problems, does it mean data is corrupted?

The question is very much the same as having a harddisk with a bunch of bad sectors.

You will be able to read everything the scratch is not touching without any problem, unless the scratch also covers the table of content.

If the scratch won't cover the ToC, then you can use the DVD and read the files normally until a file has its data in a location of the scratch. In that case, it really determines how big and deep the scratch is. DVD's are made with error correction in mind, so a small scratch may be suffering from speed reduced when the data is being read at the scratch because Error correction repairs the data on each read, but if the error correction can't do its job because the scratch is too big, there will be a read error and the read process is halted entirely.

Copying this DVD would really matter on how well the disk can still be read, and if the error correction repairs the bad data, it is possible it repairs it incorrectly. Although the file is read, it may be in a corrupted state. In that case, you could see it as read without any errors, but its still not good.

Lastly, if you do a direct DVD to DVD copy, the delay in read speed could cause the write buffer to run out in which case the burn will fail even though you technically could still copy the DVD, so copy the DVD to image first, then burn the image to prevent the write buffer to run out.