gelukkige 90ste verjaardag

Sorry, the town is Dommelen, the river is Dommel.

Don't think it is a common tradition in The Netherlands to be acknowledged with a letter of congratulations by any official authority when one turns 90 ...unless there is an other commemoration to be celebrated /acknowledged at the same time. I know that parishes might do for their elderly people .. sending a birthday card and/or flowers.
I can be mistaken... perhaps some city councils do though? You could ask via valkenswaard.nl/home (opent in een nieuw tabblad) ? FYI see>> valkenswaard.nl/bestuur-en-organisatie/feiten-en-cijfers_43187/#titel93968 (opent in een nieuw tabblad)

I have seen this for people turning 100 or above. For example: Gemeente Gemert-Bakel (municipal level) acknowlegdes a 100th birthday with congratualtions, flowers and by placing a new bench with his/her nameplate in a park or street in the township.

In almost every municipality in the Netherlands, the mayor will send a letter of congratulations to the 90-year-old.
This concerns the municipality where one lives, not where one is born.
This does not happen at provincial or national level.
You could ask the municipality of Dommelen if they would like to pay attention to this joyous event.

The village of Dommelen nowadays is part of the municipality of Valkenswaard, which you can reach on their site:
valkenswaard.nl/home/contact_42836/ (opent in een nieuw tabblad)

Dear Frank, I hope HenkD is right. I hope if you contact the municipality of Valkenswaard they will pay some attention to this special birthday for your mother. Please congratulate her from us!