If you look at pure ages then it wasn't a good look at all.
If you read the comics at the time you realise that Kitty was a very unrealistic character. No 13 year-old in real life was anything like her. And also, Colossus was written, imo, very immature for his age. So, it didn't "seem" weird, if you just read the comics and didn't pay attention to the characters ages. That doesn't mean it was a good idea at the time and Shooter was right to break it up. If she had been your typical 13 yo, then it would have seemed weird and wrong. Claremont should have just made her 16 yo initially and it wouldn't have been an issue.
After breaking up they should have just remained friends, because they aren't suited at all.