I do have iMovie 10 but do not use it so my knowledge with this version is very limited.
Have stayed with version 9.
Have you seen the quality of the MP4 file from iMovie when played in QuickTime Player?
Just to say that if the quality of the MP4 file is blurry,then so will be the DVD.
When you exported from iMovie (File >Share>File e.t.c ) what quality setting did you select?
Options are- Low,Medium,High,Best(ProRes) or Custom.
This obviously has a bearing on the output file quality.
Some people believe that the Shared -out file size has to be less than,say, 4.7GB in order to burn a DVD,
so they select a lower quality setting in order to get it all on.
In fact the file size is not the factor , but instead the movie length in time.
I would suggest you Share again and select Best (ProRes) ,you get a .mov file and bring that into iDVD.
Whilst you are at it, you could try Professional Quality in iDVD, this is the best for video between 1 and 2 hours.
Also you can find the Burn Speed setting in iDVD>Preferences>Advanced>Preferred DVD burning speed.
When I was experimenting with DVD quality burning issues some years ago I used DVD-RW rewritable
discs ( Verbatim) which saved me a fortune in wasted DVD-R discs.