When Elinor says
I am afraid...that the pleasantness of an employment does not always evince its propriety
she is basically stating that Marianne is breaking the rules of social propriety that are expected of young, unmarried ladies. Marianne allows herself to be seen in public in the constant companionship of a young man who has not formally proposed a serious relationship with her.
Within the historical context of the novel, Regency England is known for its rules of social engagement. Women are expected to be submissive, and demonstrate good manners and prudish behaviors.
Marianne is doing the exact opposite: She is boisterous, rides right next to Willoughby, and treats him like an equal. Similarly, Willoughby does not show the extreme gentlemanly and decorative mannerisms that would demonstrate any interest in marrying Marianne. Marianne assumes that Willoughby is devoted to her, and Willoughby will show how he is unreliable and unrealistic.
Therefore, the statement that Elinor makes, in basic English, means verbatim:
Just because you are enjoying this moment does not mean that what you are enjoying is the correct or proper thing to enjoy.