Not really. Her first hit, Snowbird, was in 1970 and Can-con rules were not introduced until 1971. Nor do Can-con rules explain her high level of success on U.S. pop and country charts or internationally such as Australia. 

Just looking at U.S. Billboard pop chart, Top 20 only:

1970 - Snowbird - #8
1972 - Danny's Song - #7
1973 - A Love Song - #12
1974 - You Won't See Me - #8
1978 - You Need Me - #1
1979 - I Just Fall in Love Again - #12
1979 - Broken Hearted Me - #12
1980 - Daydream Believer - #12

She had quite a few other songs place on the Billboard Top 100 - a significant achievement in the huge American market. She still continued to score within the Top 100 in the 1980s, but not reaching the Top 30. 

She did even better on the U.S. country charts (more hits, more #1s) continuing into the 1990s.