I suppose it depends on what you really enjoy doing in ACNH! I personally think it's definitely worth it!
The below could be a little bit spoiler-y if you like the surprise, as it mentions things you can unlock, but maybe it will help you decide if the expansion is worth the money for you!

Some things you can only unlock with Happy Home Paradise (to the best of my knowledge):
- Design vacation homes (including two-storey homes), for as many villagers as you'd like, including villagers you have on your island.
- Design non-home buildings, such as (in spoiler): A school, hospital and café.
- Have easier access to buying items, with a shop at Happy Home Paradise that uses a non-bell currency.
- Once the items are unlocked, it also includes being able to order items any time from a HHP catalogue.
- Work through goals, like designing x amount of homes, design a particular facility, etc.
- Unlock new DIYs for making pillars and counters.
- Unlock partition walls.
- Unlock the ability to redecorate the homes of villagers on your own home island!
- Unlock the ability to use soundscapes, different coloured lighting, and polish items for effects.