No, it is not "gross".
I'm old enough to have grown up in a home with no central heating. If you wanted a bath, you put the immersion heater on and waited two hours until there was enough hot water. If someone wanted a bath after you, they had to wait for the water to heat up again.
We weren't having daily baths or one or two showers a day. In between baths, we had daily "strip down" washes.
As people age, many of us sweat less and our skin becomes dry. It is not necessary for many elderly people to have a daily bath or shower.
As people age, they may also require a bath step, a non slip rubber mat in the bath or grab handles on the bath or on the wall. They may need a bath bench or bath seat in order to bathe safely.
I don't know how old your MIL is but unless she has a hygiene issue it is entirely her choice how often she bathes when staying in your house. And if she chooses to use bubble bath instead of soap, that is her choice, too.
I am in my early 70s and have had a a successful hip replacement. I still need the following aids in order to use a bath or shower safely and comfortably:
a non slip rubber bath mat
a small stool to get into the bath
a grab rail fixed to the bath
shower cream as soap is far too drying and makes me itchy
It really is none of your business.