There is NO mitzvah for a shadchan to lie outright. There are several aveiras, however, i.e. 1) lifnei iveir 2) gneivas daas but to name a couple. It is one thing to say a girl is pretty, when she might not be – beauty is very subjective and in the eye of the beholder. It is quite another to exaggerate about someone, when in fact they possess NONE of the qualities. I hate it when someone makes a shidduch for my daughter and will tell me,”He’s got a wonderful personality, once he gets to know you. You just need to draw him out A LITTLE.” In fact, the guy was afraid of his own shadow, said not two words on the date, and my daughter (who is very charming and easy to talk to), was at a loss. It was a waste of her time AND his.
Never say a personality problem is a little problem when it might be clinical depression. Never say he is “brilliant” when he is a mediocre student. If there is a potentially serious illness or either party is on lifelong meds, DO NOT HIDE IT! No one will trust your judgment. If the boy is short, don’t say to the short girl, “Oh you two will look so CUTE together, and you’re short, too.” She might not feel comfortable, having dated other short guys, with someone who is not a few inches taller (not necessarily tall, but at least five inches taller than herself, so she can wear heels comfortably with him).