My Mom’s Alzheimer’s is getting worse, and I wonder if planning nice days and outings is worth it anymore?

She won’t remember the event or the outing right after it’s over. Sad, but true. Is it more for you? It’s going to be very stressful & strenuous to get her anywhere.
Hugs 🤗

Squid62, as other have said, these "fun" outings will have no long-term impact for your Mom, but they're worth doing as long as you can both enjoy them in the moment. However, at some point they'll likely start to have a long-term impact on your own well-being and your patience with your Mom, i.e. when they become more torture than fun. If they're at that point, then I suggest either modifying the outings or your expectations of them, or just quit doing them.

My and my wife's experience with my Dad was that for several years, outings and even long-distance overnight trips required careful "what-if" planning and preparations, reduced expectations, lots of on-the-road flexibility, and a sense of humor. In spite of all the predictable mishaps, my Dad seemed to enjoy these frequent outings and road trips, although he didn't remember any parts of them, neither good nor bad, for more than an hour or two, certainly not for a full day. Then, at some point they became too hard on him, me and my wife, so we stopped doing them.

Kudos to you for taking your Mom on outings and best wishes for both of you as your routines have to change along with progression of the disease..

Your mom lives only in the moment. Yesterdays never were and tomorrows don't exist. Taking walks, going to the dog park or eating out somewhere have no lasting effect on her. Continue to do what you're doing especially if it makes YOU feel better. Don't worry about if the experience went good or bad, enjoy the moment with her.

My Husband LOVED going for a ride. He loved going to Sam's Club or Costco (free samples ya know)
I liked taking him because I would use the store as his walking for the day. They had large carts that he would use for stability prior to his use with a walker. And after he started using a walker the carts are tall enough that they were actually more comfortable at times than a regular walker (Until I found a larger tall one)
BUT I stopped when it was no longer SAFE for him or me to get him into the car.
I would base the outings on safety.
If it is not safe to get her into the car or out of the car then stop.
Don't worry about a destination and "going" to a place. Make the outing a ride through a park, or a county road.
One thing I learned from my Husband is .... Live in the moment....
don't think about if she remembers the ride, the park, the dinner or who was there or what was served. Does that really matter? What matters is...Did she enjoy herself during that moment?

I'm guessing that the "nice events" you're planning are more for your sake than they are for your moms. and that's ok, I certainly get that you want to have theses special memories with your mom, even though chances are great that she doesn't remember them all at this point. But the fact that your profile says that mom is living in assisted living and not memory care, leaves me to believe that she may be remembering more than you realize.
As long as you both are having a good time on these outings, I see no need to stop them. There will come a day soon enough when you will have no choice but to stop them, so enjoy them while you can. Wishing you many more fun outings with your mom.