The factory presets are not 'terrible', but factory presets are always problematic in MFX units - you'll find some you really like, some you'll think are average, and some you think are dreadful. It is the designers view of what sounds good but with their gear and are also intended to help give users an idea of what the MFX unit can do. Some users love the presets, others hate them. You also need to check whether the preset has expression pedal control as that can change it totally. Aside from personal taste ie we all have different ears, the biggest problem is inconsistency of gear and how you are using the patch ie type of style. It depends on so many variable factors that it is simply impossible to allow for when designing factory patches and very often the patch designer is trying to come up with a compromise that will be decent sounding for the majority. For example:
1. The guitar you are using - some patches will sound better with single coils and worse with humbuckers - and vice-a-versa. The pickup output will also impact on what you hear.
2. The type of headphones you have (you need good studio monitoring headphones eg Audio Technica ATHM50X, or Beyerdynamic 770 Pro - too many folk think their expensive or great sounding headphones that sound terrific when listening to you tube or mp3's or CD's should sound great with Pod Go and blame Pod Go for what they are hearing - studio monitor headphones are very different - not expensive, the two mentioned are around £120 UK, but can transform what you hear from Pod Go)
3 Your amp and how you connect it. If you are putting Pod Go through the front end, the pre-amp stage of your amp will colour tone and add distortion. If you have an FX loop, put Pod Go through the FX return for a less coloured tone
4. Your speakers - either desktop or amp will colour tone
5. The factory presets will have typically been designed in the studio using FRFR (full range, flat response) so if you are using anything else, it will sound different to what the patch creator had in mind. To prove the point watch the demo vid of Pod Go at NAAM with Phil Hindmarsh.
Now, load down Phil's patches that are available free from Line 6 Custom Tone and play through your rig? Do they sound like Phil's tones? (put aside his fabulous playing and just focus on tone). I'd be amazed if they did and you might even think the patches are 'terrible'. Even though one of my guitars is a Yamaha Pacifica 611VFM (not wholly dis-similar to his HSS Pacifica) I still had to tweak them a bit and that was with my ATHM50X headphones and going through a Headrush FRFR108 powered cab. Through a guitar amp with a different guitar, they sound totally different. So, if you want to hear the presets as intended, use studio monitoring headphones and/or an FRFR speaker.
Also, here's Paul demoing some Pod Go factory presets - do yours sound like this with your gear?
Re the part of your statement above that I've put in bold - utter and complete nonsense. Once you have an understanding of how Pod Go works, and build up your experience and knowledge of tone building (I had to go through the same learning curve, and I've been playing some 50 years and have used DSP gear for around half of that) yes of course you can. Even just changes to the cab model and/or mic type & placement, and upping the cab dB levels can transform the patch. If you've ever wondered why an IR can sound so different to the stock cabs, a big difference is the cab output level that helps give a bigger sound. Setting the frequency range to match the real speaker is another good 'trick'. I've tweaked most of the factory presets to suit inc adding snapshots and footswitch settings.
There are loads of vids on youtube by some very experienced, knowledgeable and helpful folk - it's well worth starting off with the Pod Go vids for user tips/tricks and then looking at tone-building videos - you'll find them a huge help. But here's the thing - Pod Go is NOT a simple 'plug n play' MFX unit that will give you instant tonal gratification. Just like any other MFX (and I've had Boss, Zoom, Yamaha, Vox and Line 6 units) you have to invest the time to really learn and understand how to get the best out of them - and that applies for any modelling unit out there.