I don't think Alexandra has anything against Nikolai modelling. On the contrary in fact!
There are references in this thread to Alexandra being involved in Nikolai's modeling work, albeit from the sideline. It was also a friend of Alexandra who approached her in order to offer Nikolai the chance of becoming a model. The head of the modeling agency Nikolai belongs to, is a friend of Alexandra.
The one who would perhaps like to discourage Nikolai from pursuing a professional modeling career would IMO more likely be Joachim.
ADDED:
https://www.bt.dk/royale/alexandra-var-overvaeldet-over-prins-nikolai
Alexandra was present today when he walked the catwalk.
Nikolai had a prominent role in the show.
Alexandra said via her secretary:
"It went totally fantastic. I was very overwhelmed by the fantastic show and the atmosphere was so good. I'm immensely proud of him, he did so well."
The creative head of his agency, Sune Planer said:
"The last time he opened and closed the show and today he closed it. He did it really good and closed the show and that is in itself a huge honor and very prestigious, so we are very proud of that."