Q:

Has Jenny Campbell actually invested money on Dragons' Den? 

Jenny Campbell has gained a reputation for mostly never offering to invest any money on the show, to the point that her "for that reason I'm out" catchphrase has become sort of a meme. 
Has she ever actually invested money on the show?  If yes, how many times? 

A:

Yes, on five occasions. 
Jenny Campbell's personal website lists the following five investments: drinks brand Didsbury Gin, loyalty scheme ParkingPerx, truck advertising firm DrivenMedia, energy switching service Look After My Bills, and hemp skincare producer Carun UK. 
All five of these investments were made on Dragon's Den, and as the site does not list any further investments, I find it reasonable to assume these were the only investments she made on the show. 
The website itself confirms the Didsbury Gin investment, this Business Live article confirms the ParkingPerx investment, and multiple sources (including her Wikipedia entry and this Metro article) confirm the other three investments.  Incidentally, her Wikipedia entry also notes the reputation you mentioned her having:

She built up a reputation of not making an investment in any business on the show, despite eventually investing in hemp skincare producer Carun UK, truck advertising firm Driven Media and also investing in energy switching service Look After My Bills. 

(As a final aside, Campbell has since sold her shares in Look After My Bills and Carun.) 

A:

£80k ParkingPerx
£75k Didsbury Gin
£30k Driven Media
£60k Look After My Bills
£50k Carun

£295k in total
She appeared 29 times over 2 series, so averaged just over £10k per show. 
According to Wikipedia, in terms of number of deals, this is not exceptionally low.  Sarah Willingham had 5 deals over two series (31 episodes).  
Kelly Hoppen, Piers Linney and Duncan Bannatyne had 5 or fewer deals over two consecutive series, though these series were a little shorter. 
The perception that she makes fewer offers may be true.  I could only find a single example of an offer of hers being rejected.  In which case, you could decide to frame things differently and say that statistically the offers she did make were more likely to be accepted (than for those mentioned above, at least).