Yes, there are studies that show that having a mascot increases engagement etc.
East Tennessee State University has published an Undergraduate Honours Thesis entitled: Analyzing the effects of brand mascots on social
media: Johnson City Power Board case study.
The research in this paper found that customers of Johnson City Power Board were more engaged with "Will Power" (the mascot) and more likely to be "talking about the mascot" than any of the other 'topics' on their Facebook page. Other topics included things like news stories, press releases, sponsorships, events, operations, and so on. Of course, this is talking about mascots in a social media context. However, the 24 page thesis is an interesting read on branding, mascots, etc. In its conclusion, it states:
Developing a mascot or brand character can be effective tool in building a strong brand and rapport with consumers. Mascots are useful tools in personifying a brand and developing strong, trusting relationships with consumers....Social media sites are a reliable tool for direct communication with consumers. An effective mascot can drive up communication and traffic on social media sites if used effectively.
There has also been a lot of research into the influence of food companies' brand mascots and entertainment companies' cartoon media characters on children's diet and health. One such study has a whole section on How brand mascots and media characters are used to market to children. This is an interesting read and covers 'mascots', 'characters', 'ambassadors' etc. It also has a section on How brand mascots and media characters are used to market to children.
Clearly, mascots have an impact on human behaviour, especially in younger age groups.
In fact, research has repeatedly shown that children have a significant influence on their parents purchasing decisions. In his article "Branding is no longer child's play!", published in The Journal of Consumer Marketing in 2004, Lindstrom argues:
“The BRANDchild study conducted by research institute Millward Brown among 2,000 kids aged eight to 14 across seven countries supported by data from the BrandZ study, including more than 15,000 kids of the same age across 14 countries, reveals that in up to 80 percent of all brand choices, tweens control the final decision…accounting for an astounding US$1.18 trillion per year.”
So, if you can create a mascot that 'connects' with this group, then it only follows that the mascot is indirectly influenciong parents (who are in older age groups).
And, let's not forget that tweens (aged 8-14) themselves control a significant amount of money and are also more likely to use it where they feel some sort of connection. So while it may not necessarily mean that the usage of mascots has a direct correlation on 'happiness', it is clear that creating the right mascot has a direct connection to engagement levels, and this could impact on one's perception of happiness.
After all, when you check out all the mascots used by large food companies (e.g. Ronald McDonald, all the mascots used by Kellogs for their various cereals, etc) you get a sense that their research shows a clear benefit to having mascots.
As for mascots used by software companies, the most likely place you'll see this is in video gaming software. Think of Nintendo's Mario, Luigi, Bowser etc, or of characters like Sonic the Hedgehog, Lara Croft, etc. All of these have been used to market multiple games, toys, t-shirts, etc.
For a very recent example, think of Angry Birds. The first game in the series was initially released in December 2009 for Apple's iOS devices (iPhone etc). Within two years this had been released on Android and Windows Mobile with a variety of 'games' based on the Angry Birds character. Sure, this may not have started as a mascot, but when you consider that you can buy Angry Birds clothing, pillows, blankets, board games, and that soon the Angry Birds Movie is launching, you can see how it has become a mascot for many things.
The point, however, is that all these video game characters are used by software companies to market and launch new games and a whole range of other products.
In terms of other non-game software, you've already mentioned Microsoft's Clippy, Apple did have Clarus (but I haven't seen this used in a long time), and of course there's Java's Duke; Linux's Tux and GIMP's Wilber.
Some of these have been used as characters in video games, and there's no doubting their influence in popular culture.
In terms of further reading, you may consider: