To increase the level of the tips you need to install more stylish items and get rid of the simple ones. If you purchase expensive equipment but it is simple, then the level of the tips is slightly decreased as the ratio of "stylish items/simple items" becomes lower as well.

For example, there are in your cafe all items are stylish, which means they make up 100% of all furniture and give the maximum percentage of the tips.

In case the sylish items make up only 50% of all furniture in your cafe the ratio of "stylish items/simple items" is reduced by half, as a result you will get already half of the maximum possible level of the tips.