Yes he was. This is confirmed on the Moomin Series official website:
Snufkin was inspired by Tove Jansson herself, her brother Lars, but above all Tove’s boyfriend from the beginning of the 1940’s, Atos Wirtanen.
Jansson claimed (see page at end of answer) that Snufkin's love of nature was reflective of her own fascination with the natural world (which is evident in the artistic bohemian/fairytale-like atmosphere of the Moomin's world). However, it is reiterated that Atos and Snufkin shared the most in common.
In their shared aspects there are some both visual and metaphorical:
Snufkin reminds Atos a lot with his looks – hat and pipe were common for both of them. Also backpack describes how they were coming and going all the time. Atos’ view of free life where the material assets are unnecessary is also one of the best known traits of Snufkin.
(The syntax is a bit iffy here because the website is translated from the original Finnish!)
Here is Snufkin and Wirtanen side by side where you can see a resemblance with the coat and hat:
Snufkin's characteristic 'tramp' appearance was also inspired by Wirtanen as
Atos’ view of free life where the material assets are unnecessary is also one of the best known traits of Snufkin.
The website goes on to say that Moomintroll was Jansson's alter ego and so the close friendship that Moomintroll and Snufkin have with each other mirrors Jansson and Wirtanen's relationship. This page also details her characters and their inspiration and examines Wirtanen and Snufkin amongst others.