Not at all. The issue is not the only child but how the parent raise the only child. Many parents of only children enroll their child in daycare at an early age in order for their child to develop social skills. Also parents of only children have their child enrolled in play groups with other children.
Also many only children are socially mature because they socialize with their parents and other adults. Oftentimes, these only children adopt adult mannerisms and mature far quicker than children with siblings. Many children with sibings, especially those in large families, are very insular and isolated because they mingle only within the family circle and do not have any outside friends. Whether a child is a only or non-only has nothing to do with developing social skills.
A brother and/or sister has nothing to do with whether a child develop the prerequisite social skills. There are many shy and withdrawn children who have siblings e.g. Michael Jackson. The parent is the teacher of the family and it is the parents who determine whether or not their child/children develop prerequisite social skills.
There are parents of only children who are extroverted and have a wide circle of friends and associates and their only child is exposed to the wide variety of acquaintances whereas there are parents who have multiple children who are introverted and/or insular who have a very small circle of friends or no friends and their children become very isolated and insular with poor social skills.
My father had wise words for this:nonsense and balderdash. My father had a friend who was an only child and he was the most outgoing person who willingly shared things with him. Many only children I know are highly social and universal people who are very broadminded. On the other hand, I know many loners and isolates who had siblings e.g. in elementary, there were two loners- one girl who had two siblings and one girl who had nineteen siblings-they do not like to be with people at all.