May an alien visa-holder enter the US from Europe?

It depends on some additional factors about you and the US resident. here is the actual text from the proclamation:

(a) Section 1 of this proclamation shall not apply to:

(i) any lawful permanent resident of the United States;

(ii) any alien who is the spouse of a U.S. citizen or lawful permanent resident;

(iii) any alien who is the parent or legal guardian of a U.S. citizen or lawful permanent resident, provided that the U.S. citizen or lawful permanent resident is unmarried and under the age of 21;

(iv) any alien who is the sibling of a U.S. citizen or lawful permanent resident, provided that both are unmarried and under the age of 21;

(v) any alien who is the child, foster child, or ward of a U.S. citizen or lawful permanent resident, or who is a prospective adoptee seeking to enter the United States pursuant to the IR-4 or IH-4 visa classifications;

So basically, the exception will only apply if the US resident is your spouse, or a close relative (child, sibling) as long as both you and the US resident are under 21 and unmarried. It seems that if you are the US resident's child, foster child, or ward, there is no age restriction.

It also depends on where you go in Europe. The proclamation only mentions the Schengen Area (which doesn't comprise all European countries):

For purposes of this proclamation, the Schengen Area comprises 26 European states: Austria, Belgium, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Italy, Latvia, Liechtenstein, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Slovakia, Slovenia, Spain, Sweden, and Switzerland.

Also you might want to consult https://www.iatatravelcentre.com/international-travel-document-news/1580226297.htm for more up-to-date information (e.g. the ban currently includes Ireland):

Published 14.05.2020

1. Passengers who have transited or have been in Austria, Belgium, China (People's Rep.), Czechia, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Iran, Ireland (Rep.), Italy, Latvia, Liechtenstein, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Slovakia, Slovenia, Spain, Sweden, Switzerland or in the United Kingdom (excluding overseas territories outside of Europe), in the past 14 days are not allowed to enter the USA. -This does not apply to:
   • nationals and permanent residents of the USA;
   • spouses of nationals and of permanent residents of the USA;
   • parents/legal guardians of an unmarried and younger than 21 years national or permanent resident of the USA;
   • the unmarried and younger than 21 years brother or sister of a national or permanent resident of the USA, who is unmarried and younger than 21 years;
   • the child/foster child/ward of a national or permanent resident of the USA;
   • passengers with the following visas: A-1, A-2, C-1, C-1/D, C-2, C-3, D, E-1, G-1, G-2, G-3, G-4, IR-4, IH-4, NATO-1 through NATO-4, or NATO-6 visa;
   • members of the U.S. Armed Forces, spouses, and children of members of the U.S. Armed Forces;
   • passengers with evidence of traveling at the invitation of the USA government for a purpose related to the containment/mitigation of the Coronavirus (COVID-19);
   • passengers with documents issued by the U.S. Department of Homeland Security, U.S. Customs and Border Protection, or U.S. Department of State indicating that the passenger is exempt from the restriction;
   • B1 crew crewmembers that are engaged in lightering, Outer Continental Shelf (OCS) activity, wind farm activity, private air/sea crew and other similar crewmember actives.
2. Passengers who have been in one of the countries listed in bullet number 1 in the past 14 days but are exempt from the restriction to enter the USA, must arrive at one of the following airports: Atlanta (ATL), Boston (BOS), Chicago (ORD), Dallas, (DFW), Detroit (DTW), Honolulu (HNL), Los Angeles (LAX), Miami (MIA), New York (JFK or EWR), San Francisco (SFO), Seattle (SEA) and Washington (IAD).
3. Passengers who have been in Austria, Belgium, China (People's Rep.), Czechia, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Iran, Ireland (Rep.), Italy, Latvia, Liechtenstein, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Slovakia, Slovenia, Spain, Sweden, Switzerland or United Kingdom (excluding overseas territories outside of Europe) in the past 14 days but are exempt from the restriction to enter the USA, must self-quarantine for 14 days once they reach their final destination.