As with all countries with a Tropical climate, you will find mosquitoes, so it’s best to bring some repellent with you, especially as it can be hard to source on the Island itself.
Most areas, especially tourist areas, are treated for mosquitoes, and Cuba does use the sterile insect technique (SIT) in cooperation with the United Nations (UN), Food and Agriculture Organization (FAO), which after a trial in 2022 in Havana saw a 90% reduction of mosquitoes in the city. This form of control is also seen as the most environmentally friendly approach to dealing with these troublesome insects.
Havana and Varadero are the least affected areas for mosquitoes, whilst areas with more lush vegetation like Vinales, and other national parks, or where hotels are built over water like some in Cayo Coco for example, are affected more.
In short, as long as you bring some repellent there’s no reason why mosquitoes should stop you from enjoying your stay on the beautiful Island that is Cuba.