Stella'sMum - my little girl who is now 18mo learnt to crawl this exact way you describe also!! I kept wondering when would she ever crawl 'properly' but turns out, that was just the way she wanted to crawl. I think as long as they are crawling (doesn't matter how!) then that's a good thing. She bum shuffled first to get around between 8-10.5mths (oh so cute to watch!!), and then 10.5mths she crawled and walked the day she turned 14mths exactly. She learnt to walk very quickly from that day on and ever since she's been walking, hasn't bothered crawling!! She never really liked crawling much I don't think!! And I think that's fine.
Very rarely will she be on her hands and knees and crawl, why crawl when you can walk?!?! but when she does, she does seem to do it properly these days which is good rather than dragging that leg hehe.

And my little girl is bright, biggest chatterbox EVER!, and ahead of many of her peers in lots of areas so don't worry your little one is just fine