Isn't St Mungo's a charity that looks after homeless people?
It is perfectly reasonable to say it's a name in a Celtic language that means lovable, because sites that are often authoritative, like Wikipedia, do repeat vague etymologies like:
"Particularly in Scotland, he is known by the pet name Mungo, possibly derived from the Cumbric equivalent of the Welsh: fy nghu 'my dear (one)'.[4] The Mungo pet name or hypocorism has a Gaelic parallel in the form Mo Choe or Mo Cha, under which guise Kentigern appears in Kirkmahoe, for example, in Dumfriesshire, which appears as 'ecclesia Sancti Kentigerni' in the Arbroath Liber in 1321. An ancient church in Bromfield, Cumbria is named after him, as are Crosthwaite Parish Church and some other churches in the northern part of Cumbria, for example St Mungo's Church, Dearham."
What this paragraph says is that the person known as Saint Kentigern had two nicknames "Mungo" and "Mo Choe". "Mo" means "my", and in Irish it lenites (softens, shaves) the initial consonant of the next word. In Modern Welsh "my" is "fy", and it nasalises the initial consonant of the next word - think of French "mon" and the n getting stuck onto the next word. In modern English "our" is used to make a nickname "Our Johnny", "Our Mary" and in this case "Our Co", in those days it was "My Co".
When you think about it like that, you realise the emphasis is on the "go", so the name is "MunGO", not "MUNgo".
It is a nickname so you could use any name beginning Co like Conaire, Colm and have Mungo as a nickname. Similar (maybe same) names which avoid association with the perjorative "mong" include Munno and Mocha.
Sorry for long rambling unedited post I should be writing something else.....